diff --git a/src/library/archive_entry.cr b/src/library/archive_entry.cr
new file mode 100644
index 0000000..cd63f17
--- /dev/null
+++ b/src/library/archive_entry.cr
@@ -0,0 +1,111 @@
+require "yaml"
+
+require "./entry"
+
+class ArchiveEntry < Entry
+  include YAML::Serializable
+
+  getter zip_path : String
+
+  def initialize(@zip_path, @book)
+    storage = Storage.default
+    @path = @zip_path
+    @encoded_path = URI.encode @zip_path
+    @title = File.basename @zip_path, File.extname @zip_path
+    @encoded_title = URI.encode @title
+    @size = (File.size @zip_path).humanize_bytes
+    id = storage.get_entry_id @zip_path, File.signature(@zip_path)
+    if id.nil?
+      id = random_str
+      storage.insert_entry_id({
+        path:      @zip_path,
+        id:        id,
+        signature: File.signature(@zip_path).to_s,
+      })
+    end
+    @id = id
+    @mtime = File.info(@zip_path).modification_time
+
+    unless File.readable? @zip_path
+      @err_msg = "File #{@zip_path} is not readable."
+      Logger.warn "#{@err_msg} Please make sure the " \
+                  "file permission is configured correctly."
+      return
+    end
+
+    archive_exception = validate_archive @zip_path
+    unless archive_exception.nil?
+      @err_msg = "Archive error: #{archive_exception}"
+      Logger.warn "Unable to extract archive #{@zip_path}. " \
+                  "Ignoring it. #{@err_msg}"
+      return
+    end
+
+    file = ArchiveFile.new @zip_path
+    @pages = file.entries.count do |e|
+      SUPPORTED_IMG_TYPES.includes? \
+        MIME.from_filename? e.filename
+    end
+    file.close
+  end
+
+  private def sorted_archive_entries
+    ArchiveFile.open @zip_path do |file|
+      entries = file.entries
+        .select { |e|
+          SUPPORTED_IMG_TYPES.includes? \
+            MIME.from_filename? e.filename
+        }
+        .sort! { |a, b|
+          compare_numerically a.filename, b.filename
+        }
+      yield file, entries
+    end
+  end
+
+  def read_page(page_num)
+    raise "Unreadble archive. #{@err_msg}" if @err_msg
+    img = nil
+    begin
+      sorted_archive_entries do |file, entries|
+        page = entries[page_num - 1]
+        data = file.read_entry page
+        if data
+          img = Image.new data, MIME.from_filename(page.filename),
+            page.filename, data.size
+        end
+      end
+    rescue e
+      Logger.warn "Unable to read page #{page_num} of #{@zip_path}. Error: #{e}"
+    end
+    img
+  end
+
+  def page_dimensions
+    sizes = [] of Hash(String, Int32)
+    sorted_archive_entries do |file, entries|
+      entries.each_with_index do |e, i|
+        begin
+          data = file.read_entry(e).not_nil!
+          size = ImageSize.get data
+          sizes << {
+            "width"  => size.width,
+            "height" => size.height,
+          }
+        rescue e
+          Logger.warn "Failed to read page #{i} of entry #{zip_path}. #{e}"
+          sizes << {"width" => 1000_i32, "height" => 1000_i32}
+        end
+      end
+    end
+    sizes
+  end
+
+  def examine : Bool
+    File.exists? @zip_path
+  end
+
+  def self.is_valid?(path : String) : Bool
+    is_supported_file path
+  end
+end
diff --git a/src/library/cache.cr b/src/library/cache.cr
index 10e4f60..f35af8b 100644
--- a/src/library/cache.cr
+++ b/src/library/cache.cr
@@ -76,8 +76,8 @@ class SortedEntriesCacheEntry < CacheEntry(Array(String), Array(Entry))
                    entries : Array(Entry), opt : SortOptions?)
     entries_sig = Digest::SHA1.hexdigest (entries.map &.id).to_s
     user_context = opt && opt.method == SortMethod::Progress ? username : ""
-    sig = Digest::SHA1.hexdigest (book_id + entries_sig + user_context +
-                                  (opt ? opt.to_tuple.to_s : "nil"))
+    sig = Digest::SHA1.hexdigest(book_id + entries_sig + user_context +
+                                 (opt ? opt.to_tuple.to_s : "nil"))
     "#{sig}:sorted_entries"
   end
 end
@@ -101,8 +101,8 @@ class SortedTitlesCacheEntry < CacheEntry(Array(String), Array(Title))
   def self.gen_key(username : String, titles : Array(Title), opt : SortOptions?)
     titles_sig = Digest::SHA1.hexdigest (titles.map &.id).to_s
     user_context = opt && opt.method == SortMethod::Progress ? username : ""
-    sig = Digest::SHA1.hexdigest (titles_sig + user_context +
-                                  (opt ? opt.to_tuple.to_s : "nil"))
+    sig = Digest::SHA1.hexdigest(titles_sig + user_context +
+                                 (opt ? opt.to_tuple.to_s : "nil"))
     "#{sig}:sorted_titles"
   end
 end
diff --git a/src/library/dir_entry.cr b/src/library/dir_entry.cr
new file mode 100644
index 0000000..0ce4e71
--- /dev/null
+++ b/src/library/dir_entry.cr
@@ -0,0 +1,132 @@
+require "yaml"
+
+require "./entry"
+
+class DirEntry < Entry
+  include YAML::Serializable
+
+  getter dir_path : String
+
+  @[YAML::Field(ignore: true)]
+  @sorted_files : Array(String)?
+
+  @signature : String
+
+  def initialize(@dir_path, @book)
+    storage = Storage.default
+    @path = @dir_path
+    @encoded_path = URI.encode @dir_path
+    @title = File.basename @dir_path
+    @encoded_title = URI.encode @title
+
+    unless File.readable? @dir_path
+      @err_msg = "Directory #{@dir_path} is not readable."
+      Logger.warn "#{@err_msg} Please make sure the " \
+                  "file permission is configured correctly."
+      return
+    end
+
+    unless DirEntry.is_valid? @dir_path
+      @err_msg = "Directory #{@dir_path} is not valid directory entry."
+      Logger.warn "#{@err_msg} Please make sure the " \
+                  "directory has valid images."
+      return
+    end
+
+    size_sum = 0
+    sorted_files.each do |file_path|
+      size_sum += File.size file_path
+    end
+    @size = size_sum.humanize_bytes
+
+    @signature = Dir.directory_entry_signature @dir_path
+    id = storage.get_entry_id @dir_path, @signature
+    if id.nil?
+      id = random_str
+      storage.insert_entry_id({
+        path:      @dir_path,
+        id:        id,
+        signature: @signature,
+      })
+    end
+    @id = id
+
+    @mtime = sorted_files.map do |file_path|
+      File.info(file_path).modification_time
+    end.max
+    @pages = sorted_files.size
+  end
+
+  def read_page(page_num)
+    img = nil
+    begin
+      files = sorted_files
+      file_path = files[page_num - 1]
+      data = File.read(file_path).to_slice
+      if data
+        img = Image.new data, MIME.from_filename(file_path),
+          File.basename(file_path), data.size
+      end
+    rescue e
+      Logger.warn "Unable to read page #{page_num} of #{@dir_path}. Error: #{e}"
+    end
+    img
+  end
+
+  def page_dimensions
+    sizes = [] of Hash(String, Int32)
+    sorted_files.each_with_index do |path, i|
+      data = File.read(path).to_slice
+      begin
+        data.not_nil!
+        size = ImageSize.get data
+        sizes << {
+          "width"  => size.width,
+          "height" => size.height,
+        }
+      rescue e
+        Logger.warn "Failed to read page #{i} of entry #{@dir_path}. #{e}"
+        sizes << {"width" => 1000_i32, "height" => 1000_i32}
+      end
+    end
+    sizes
+  end
+
+  def examine : Bool
+    existence = File.exists? @dir_path
+    return false unless existence
+    files = DirEntry.image_files @dir_path
+    signature = Dir.directory_entry_signature @dir_path
+    existence = files.size > 0 && @signature == signature
+    @sorted_files = nil unless existence
+
+    # For more efficient, update a directory entry with new property
+    # and return true like Title.examine
+    existence
+  end
+
+  def sorted_files
+    cached_sorted_files = @sorted_files
+    return cached_sorted_files if cached_sorted_files
+    @sorted_files = DirEntry.sorted_image_files @dir_path
+    @sorted_files.not_nil!
+  end
+
+  def self.image_files(dir_path)
+    Dir.entries(dir_path)
+      .reject(&.starts_with? ".")
+      .map { |fn| File.join dir_path, fn }
+      .select { |fn| is_supported_image_file fn }
+      .reject { |fn| File.directory? fn }
+      .select { |fn| File.readable? fn }
+  end
+
+  def self.sorted_image_files(dir_path)
+    self.image_files(dir_path)
+      .sort { |a, b| compare_numerically a, b }
+  end
+
+  def self.is_valid?(path : String) : Bool
+    image_files(path).size > 0
+  end
+end
diff --git a/src/library/entry.cr b/src/library/entry.cr
index dd50ed3..16666ea 100644
--- a/src/library/entry.cr
+++ b/src/library/entry.cr
@@ -1,66 +1,55 @@
 require "image_size"
-require "yaml"
 
-class Entry
-  include YAML::Serializable
+private def node_has_key(node : YAML::Nodes::Mapping, key : String)
+  node.nodes
+    .map_with_index { |n, i| {n, i} }
+    .select(&.[1].even?)
+    .map(&.[0])
+    .select(YAML::Nodes::Scalar)
+    .map(&.as(YAML::Nodes::Scalar).value)
+    .includes? key
+end
 
-  getter zip_path : String, book : Title, title : String,
-    size : String, pages : Int32, id : String, encoded_path : String,
-    encoded_title : String, mtime : Time, err_msg : String?
+abstract class Entry
+  getter id : String, book : Title, title : String, path : String,
+    size : String, pages : Int32, mtime : Time,
+    encoded_path : String, encoded_title : String, err_msg : String?
 
-  @[YAML::Field(ignore: true)]
-  @sort_title : String?
+  def initialize(
+    @id, @title, @book, @path,
+    @size, @pages, @mtime,
+    @encoded_path, @encoded_title, @err_msg
+  )
+  end
 
-  def initialize(@zip_path, @book)
-    storage = Storage.default
-    @encoded_path = URI.encode @zip_path
-    @title = File.basename @zip_path, File.extname @zip_path
-    @encoded_title = URI.encode @title
-    @size = (File.size @zip_path).humanize_bytes
-    id = storage.get_entry_id @zip_path, File.signature(@zip_path)
-    if id.nil?
-      id = random_str
-      storage.insert_entry_id({
-        path:      @zip_path,
-        id:        id,
-        signature: File.signature(@zip_path).to_s,
-      })
+  def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
+    unless node.is_a? YAML::Nodes::Mapping
+      raise "Unexpected node type in YAML"
     end
-    @id = id
-    @mtime = File.info(@zip_path).modification_time
-
-    unless File.readable? @zip_path
-      @err_msg = "File #{@zip_path} is not readable."
-      Logger.warn "#{@err_msg} Please make sure the " \
-                  "file permission is configured correctly."
-      return
+    # Doing YAML::Any.new(ctx, node) here causes a weird error, so
+    #   instead we are using a more hacky approach (see `node_has_key`).
+    # TODO: Use a more elegant approach
+    if node_has_key node, "zip_path"
+      ArchiveEntry.new ctx, node
+    elsif node_has_key node, "dir_path"
+      DirEntry.new ctx, node
+    else
+      raise "Unknown entry found in YAML cache. Try deleting the " \
+            "`library.yml.gz` file"
     end
-
-    archive_exception = validate_archive @zip_path
-    unless archive_exception.nil?
-      @err_msg = "Archive error: #{archive_exception}"
-      Logger.warn "Unable to extract archive #{@zip_path}. " \
-                  "Ignoring it. #{@err_msg}"
-      return
-    end
-
-    file = ArchiveFile.new @zip_path
-    @pages = file.entries.count do |e|
-      SUPPORTED_IMG_TYPES.includes? \
-        MIME.from_filename? e.filename
-    end
-    file.close
   end
 
   def build_json(*, slim = false)
     JSON.build do |json|
       json.object do
-        {% for str in %w(zip_path title size id) %}
-        json.field {{str}}, @{{str.id}}
+        {% for str in %w(path title size id) %}
+        json.field {{str}}, {{str.id}}
       {% end %}
         if err_msg
           json.field "err_msg", err_msg
         end
+        json.field "zip_path", path # for API backward compatability
+        json.field "path", path
         json.field "title_id", @book.id
         json.field "title_title", @book.title
         json.field "sort_title", sort_title
@@ -74,6 +63,9 @@ class Entry
     end
   end
 
+  @[YAML::Field(ignore: true)]
+  @sort_title : String?
+
   def sort_title
     sort_title_cached = @sort_title
     return sort_title_cached if sort_title_cached
@@ -131,58 +123,6 @@ class Entry
     url
   end
 
-  private def sorted_archive_entries
-    ArchiveFile.open @zip_path do |file|
-      entries = file.entries
-        .select { |e|
-          SUPPORTED_IMG_TYPES.includes? \
-            MIME.from_filename? e.filename
-        }
-        .sort! { |a, b|
-          compare_numerically a.filename, b.filename
-        }
-      yield file, entries
-    end
-  end
-
-  def read_page(page_num)
-    raise "Unreadble archive. #{@err_msg}" if @err_msg
-    img = nil
-    begin
-      sorted_archive_entries do |file, entries|
-        page = entries[page_num - 1]
-        data = file.read_entry page
-        if data
-          img = Image.new data, MIME.from_filename(page.filename),
-            page.filename, data.size
-        end
-      end
-    rescue e
-      Logger.warn "Unable to read page #{page_num} of #{@zip_path}. Error: #{e}"
-    end
-    img
-  end
-
-  def page_dimensions
-    sizes = [] of Hash(String, Int32)
-    sorted_archive_entries do |file, entries|
-      entries.each_with_index do |e, i|
-        begin
-          data = file.read_entry(e).not_nil!
-          size = ImageSize.get data
-          sizes << {
-            "width"  => size.width,
-            "height" => size.height,
-          }
-        rescue e
-          Logger.warn "Failed to read page #{i} of entry #{zip_path}. #{e}"
-          sizes << {"width" => 1000_i32, "height" => 1000_i32}
-        end
-      end
-    end
-    sizes
-  end
-
   def next_entry(username)
     entries = @book.sorted_entries username
     idx = entries.index self
@@ -197,20 +137,6 @@ class Entry
     entries[idx - 1]
   end
 
-  def date_added
-    date_added = nil
-    TitleInfo.new @book.dir do |info|
-      info_da = info.date_added[@title]?
-      if info_da.nil?
-        date_added = info.date_added[@title] = ctime @zip_path
-        info.save
-      else
-        date_added = info_da
-      end
-    end
-    date_added.not_nil! # is it ok to set not_nil! here?
-  end
-
   # For backward backward compatibility with v0.1.0, we save entry titles
   #   instead of IDs in info.json
   def save_progress(username, page)
@@ -290,7 +216,7 @@ class Entry
       end
       Storage.default.save_thumbnail @id, img
     rescue e
-      Logger.warn "Failed to generate thumbnail for file #{@zip_path}. #{e}"
+      Logger.warn "Failed to generate thumbnail for file #{path}. #{e}"
     end
 
     img
@@ -299,4 +225,34 @@ class Entry
   def get_thumbnail : Image?
     Storage.default.get_thumbnail @id
   end
+
+  def date_added : Time
+    date_added = Time::UNIX_EPOCH
+    TitleInfo.new @book.dir do |info|
+      info_da = info.date_added[@title]?
+      if info_da.nil?
+        date_added = info.date_added[@title] = ctime path
+        info.save
+      else
+        date_added = info_da
+      end
+    end
+    date_added
+  end
+
+  # Hack to have abstract class methods
+  # https://github.com/crystal-lang/crystal/issues/5956
+  private module ClassMethods
+    abstract def is_valid?(path : String) : Bool
+  end
+
+  macro inherited
+    extend ClassMethods
+  end
+
+  abstract def read_page(page_num)
+
+  abstract def page_dimensions
+
+  abstract def examine : Bool?
 end
diff --git a/src/library/title.cr b/src/library/title.cr
index e3d79d5..e9873f2 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -49,13 +49,18 @@ class Title
       path = File.join dir, fn
       if File.directory? path
         title = Title.new path, @id, cache
-        next if title.entries.size == 0 && title.titles.size == 0
-        Library.default.title_hash[title.id] = title
-        @title_ids << title.id
+        unless title.entries.size == 0 && title.titles.size == 0
+          Library.default.title_hash[title.id] = title
+          @title_ids << title.id
+        end
+        if DirEntry.is_valid? path
+          entry = DirEntry.new path, self
+          @entries << entry if entry.pages > 0 || entry.err_msg
+        end
         next
       end
       if is_supported_file path
-        entry = Entry.new path, self
+        entry = ArchiveEntry.new path, self
         @entries << entry if entry.pages > 0 || entry.err_msg
       end
     end
@@ -127,12 +132,12 @@ class Title
 
     previous_entries_size = @entries.size
     @entries.select! do |entry|
-      existence = File.exists? entry.zip_path
+      existence = entry.examine
       Fiber.yield
       context["deleted_entry_ids"] << entry.id unless existence
       existence
     end
-    remained_entry_zip_paths = @entries.map &.zip_path
+    remained_entry_paths = @entries.map &.path
 
     is_titles_added = false
     is_entries_added = false
@@ -140,29 +145,43 @@ class Title
       next if fn.starts_with? "."
       path = File.join dir, fn
       if File.directory? path
+        unless remained_entry_paths.includes? path
+          if DirEntry.is_valid? path
+            entry = DirEntry.new path, self
+            if entry.pages > 0 || entry.err_msg
+              @entries << entry
+              is_entries_added = true
+              context["deleted_entry_ids"].select! do |deleted_entry_id|
+                entry.id != deleted_entry_id
+              end
+            end
+          end
+        end
+
         next if remained_title_dirs.includes? path
         title = Title.new path, @id, context["cached_contents_signature"]
-        next if title.entries.size == 0 && title.titles.size == 0
-        Library.default.title_hash[title.id] = title
-        @title_ids << title.id
-        is_titles_added = true
+        unless title.entries.size == 0 && title.titles.size == 0
+          Library.default.title_hash[title.id] = title
+          @title_ids << title.id
+          is_titles_added = true
 
-        # We think they are removed, but they are here!
-        # Cancel reserved jobs
-        revival_title_ids = [title.id] + title.deep_titles.map &.id
-        context["deleted_title_ids"].select! do |deleted_title_id|
-          !(revival_title_ids.includes? deleted_title_id)
-        end
-        revival_entry_ids = title.deep_entries.map &.id
-        context["deleted_entry_ids"].select! do |deleted_entry_id|
-          !(revival_entry_ids.includes? deleted_entry_id)
+          # We think they are removed, but they are here!
+          # Cancel reserved jobs
+          revival_title_ids = [title.id] + title.deep_titles.map &.id
+          context["deleted_title_ids"].select! do |deleted_title_id|
+            !(revival_title_ids.includes? deleted_title_id)
+          end
+          revival_entry_ids = title.deep_entries.map &.id
+          context["deleted_entry_ids"].select! do |deleted_entry_id|
+            !(revival_entry_ids.includes? deleted_entry_id)
+          end
         end
 
         next
       end
       if is_supported_file path
-        next if remained_entry_zip_paths.includes? path
-        entry = Entry.new path, self
+        next if remained_entry_paths.includes? path
+        entry = ArchiveEntry.new path, self
         if entry.pages > 0 || entry.err_msg
           @entries << entry
           is_entries_added = true
@@ -627,7 +646,7 @@ class Title
 
     @entries.each do |e|
       next if da.has_key? e.title
-      da[e.title] = ctime e.zip_path
+      da[e.title] = ctime e.path
     end
 
     TitleInfo.new @dir do |info|
diff --git a/src/library/types.cr b/src/library/types.cr
index 973aa5e..d6a014f 100644
--- a/src/library/types.cr
+++ b/src/library/types.cr
@@ -1,13 +1,3 @@
-SUPPORTED_IMG_TYPES = %w(
-  image/jpeg
-  image/png
-  image/webp
-  image/apng
-  image/avif
-  image/gif
-  image/svg+xml
-)
-
 enum SortMethod
   Auto
   Title
diff --git a/src/routes/api.cr b/src/routes/api.cr
index e664b28..89b4a30 100644
--- a/src/routes/api.cr
+++ b/src/routes/api.cr
@@ -40,7 +40,7 @@ struct APIRouter
     Koa.schema "entry", {
       "pages" => Int32,
       "mtime" => Int64,
-    }.merge(s %w(zip_path title size id title_id display_name cover_url)),
+    }.merge(s %w(zip_path path title size id title_id display_name cover_url)),
       desc: "An entry in a book"
 
     Koa.schema "title", {
@@ -142,8 +142,13 @@ struct APIRouter
           env.response.status_code = 304
           ""
         else
+          if entry.is_a? DirEntry
+            cache_control = "no-cache, max-age=86400"
+          else
+            cache_control = "public, max-age=86400"
+          end
           env.response.headers["ETag"] = e_tag
-          env.response.headers["Cache-Control"] = "public, max-age=86400"
+          env.response.headers["Cache-Control"] = cache_control
           send_img env, img
         end
       rescue e
@@ -1138,15 +1143,24 @@ struct APIRouter
         entry = title.get_entry eid
         raise "Entry ID `#{eid}` of `#{title.title}` not found" if entry.nil?
 
-        file_hash = Digest::SHA1.hexdigest (entry.zip_path + entry.mtime.to_s)
+        if entry.is_a? DirEntry
+          file_hash = Digest::SHA1.hexdigest(entry.path + entry.mtime.to_s + entry.size)
+        else
+          file_hash = Digest::SHA1.hexdigest(entry.path + entry.mtime.to_s)
+        end
         e_tag = "W/#{file_hash}"
         if e_tag == prev_e_tag
           env.response.status_code = 304
           send_text env, ""
         else
           sizes = entry.page_dimensions
+          if entry.is_a? DirEntry
+            cache_control = "no-cache, max-age=86400"
+          else
+            cache_control = "public, max-age=86400"
+          end
           env.response.headers["ETag"] = e_tag
-          env.response.headers["Cache-Control"] = "public, max-age=86400"
+          env.response.headers["Cache-Control"] = cache_control
           send_json env, {
             "success"    => true,
             "dimensions" => sizes,
@@ -1172,7 +1186,7 @@ struct APIRouter
         title = (Library.default.get_title env.params.url["tid"]).not_nil!
         entry = (title.get_entry env.params.url["eid"]).not_nil!
 
-        send_attachment env, entry.zip_path
+        send_attachment env, entry.path
       rescue e
         Logger.error e
         env.response.status_code = 404
diff --git a/src/routes/reader.cr b/src/routes/reader.cr
index 40b86aa..f76dc2d 100644
--- a/src/routes/reader.cr
+++ b/src/routes/reader.cr
@@ -53,6 +53,7 @@ struct ReaderRouter
         render "src/views/reader.html.ecr"
       rescue e
         Logger.error e
+        Logger.debug e.backtrace?
         env.response.status_code = 404
       end
     end
diff --git a/src/util/signature.cr b/src/util/signature.cr
index 5ca3e14..8d2b961 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -19,7 +19,7 @@ class File
   #   information as long as the above changes do not happen together with
   #   a file/folder rename, with no library scan in between.
   def self.signature(filename) : UInt64
-    if is_supported_file filename
+    if ArchiveEntry.is_valid?(filename) || is_supported_image_file(filename)
       File.info(filename).inode
     else
       0u64
@@ -64,10 +64,11 @@ class Dir
         path = File.join dirname, fn
         if File.directory? path
           signatures << Dir.contents_signature path, cache
+          signatures << fn if DirEntry.is_valid? path
         else
           # Only add its signature value to `signatures` when it is a
           #   supported file
-          signatures << fn if is_supported_file fn
+          signatures << fn if ArchiveEntry.is_valid? fn
         end
         Fiber.yield
       end
@@ -76,4 +77,19 @@ class Dir
     cache[dirname] = hash
     hash
   end
+
+  def self.directory_entry_signature(dirname, cache = {} of String => String)
+    return cache[dirname + "?entry"] if cache[dirname + "?entry"]?
+    Fiber.yield
+    signatures = [] of String
+    image_files = DirEntry.sorted_image_files dirname
+    if image_files.size > 0
+      image_files.each do |path|
+        signatures << File.signature(path).to_s
+      end
+    end
+    hash = Digest::SHA1.hexdigest(signatures.join)
+    cache[dirname + "?entry"] = hash
+    hash
+  end
 end
diff --git a/src/util/util.cr b/src/util/util.cr
index e7b1b1a..e08bd9d 100644
--- a/src/util/util.cr
+++ b/src/util/util.cr
@@ -3,6 +3,15 @@ ENTRIES_IN_HOME_SECTIONS = 8
 UPLOAD_URL_PREFIX        = "/uploads"
 STATIC_DIRS              = %w(/css /js /img /webfonts /favicon.ico /robots.txt)
 SUPPORTED_FILE_EXTNAMES  = [".zip", ".cbz", ".rar", ".cbr"]
+SUPPORTED_IMG_TYPES      = %w(
+  image/jpeg
+  image/png
+  image/webp
+  image/apng
+  image/avif
+  image/gif
+  image/svg+xml
+)
 
 def random_str
   UUID.random.to_s.gsub "-", ""
@@ -49,6 +58,10 @@ def is_supported_file(path)
   SUPPORTED_FILE_EXTNAMES.includes? File.extname(path).downcase
 end
 
+def is_supported_image_file(path)
+  SUPPORTED_IMG_TYPES.includes? MIME.from_filename? path
+end
+
 struct Int
   def or(other : Int)
     if self == 0
diff --git a/src/views/opds/title.xml.ecr b/src/views/opds/title.xml.ecr
index b159687..1d82490 100644
--- a/src/views/opds/title.xml.ecr
+++ b/src/views/opds/title.xml.ecr
@@ -29,7 +29,7 @@
         <link rel="http://opds-spec.org/image" href="<%= e.cover_url %>" />
         <link rel="http://opds-spec.org/image/thumbnail" href="<%= e.cover_url %>" />
 
-        <link rel="http://opds-spec.org/acquisition" href="<%= base_url %>api/download/<%= e.book.id %>/<%= e.id %>" title="Read" type="<%= MIME.from_filename e.zip_path %>" />
+        <link rel="http://opds-spec.org/acquisition" href="<%= base_url %>api/download/<%= e.book.id %>/<%= e.id %>" title="Read" type="<%= MIME.from_filename e.path %>" />
 
         <link type="text/html" rel="alternate" title="Read in Mango" href="<%= base_url %>reader/<%= e.book.id %>/<%= e.id %>" />
         <link type="text/html" rel="alternate" title="Open in Mango" href="<%= base_url %>book/<%= e.book.id %>" />
diff --git a/src/views/reader-error.html.ecr b/src/views/reader-error.html.ecr
index 62a80fc..ad3580f 100644
--- a/src/views/reader-error.html.ecr
+++ b/src/views/reader-error.html.ecr
@@ -5,7 +5,7 @@
       <div>
         <h3 class="uk-modal-title uk-margin-remove-top">Error</h3>
       </div>
-      <p class="uk-text-meta uk-margin-remove-bottom"><%= entry.zip_path %></p>
+      <p class="uk-text-meta uk-margin-remove-bottom"><%= entry.path %></p>
       <p class="uk-text-meta uk-margin-remove-top"><%= entry.err_msg %></p>
     </div>
     <div class="uk-modal-body">
diff --git a/src/views/reader.html.ecr b/src/views/reader.html.ecr
index feac115..19e2b19 100644
--- a/src/views/reader.html.ecr
+++ b/src/views/reader.html.ecr
@@ -67,7 +67,7 @@
         <button class="uk-modal-close-default" type="button" uk-close></button>
         <div class="uk-modal-header">
           <h3 class="uk-modal-title break-word"><%= entry.display_name %></h3>
-          <p class="uk-text-meta uk-margin-remove-bottom break-word"><%= entry.zip_path %></p>
+          <p class="uk-text-meta uk-margin-remove-bottom break-word"><%= entry.path %></p>
         </div>
         <div class="uk-modal-body">
           <div class="uk-margin">