diff --git a/src/library/entry.cr b/src/library/entry.cr
index 0a08246..43fbb23 100644
--- a/src/library/entry.cr
+++ b/src/library/entry.cr
@@ -1,6 +1,9 @@
 require "image_size"
+require "yaml"
 
 class Entry
+  include YAML::Serializable
+
   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?
diff --git a/src/library/library.cr b/src/library/library.cr
index 508a274..ce1a8fc 100644
--- a/src/library/library.cr
+++ b/src/library/library.cr
@@ -1,12 +1,41 @@
 class Library
+  include YAML::Serializable
+
   getter dir : String, title_ids : Array(String),
     title_hash : Hash(String, Title)
 
   use_default
 
-  def initialize
-    register_mime_types
+  def save_instance
+    path = Config.current.library_path
+    instance_file_path = File.join path, "library.yml.gz"
+    Logger.debug "Caching library to #{instance_file_path}"
 
+    writer = Compress::Gzip::Writer.new instance_file_path,
+      Compress::Gzip::BEST_COMPRESSION
+    writer.write self.to_yaml.to_slice
+    writer.close
+  end
+
+  def self.load_instance
+    dir = Config.current.library_path
+    return unless Dir.exists? dir
+    instance_file_path = File.join dir, "library.yml.gz"
+    return unless File.exists? instance_file_path
+
+    Logger.debug "Loading cached library from #{instance_file_path}"
+
+    begin
+      Compress::Gzip::Reader.open instance_file_path do |content|
+        @@default = Library.from_yaml content
+      end
+      Library.default.register_jobs
+    rescue e
+      Logger.error e
+    end
+  end
+
+  def initialize
     @dir = Config.current.library_path
     # explicitly initialize @titles to bypass the compiler check. it will
     #   be filled with actual Titles in the `scan` call below
@@ -16,6 +45,12 @@ class Library
     @entries_count = 0
     @thumbnails_count = 0
 
+    register_jobs
+  end
+
+  protected def register_jobs
+    register_mime_types
+
     scan_interval = Config.current.scan_interval_minutes
     if scan_interval < 1
       scan
@@ -25,7 +60,7 @@ class Library
           start = Time.local
           scan
           ms = (Time.local - start).total_milliseconds
-          Logger.info "Scanned #{@title_ids.size} titles in #{ms}ms"
+          Logger.debug "Library initialized in #{ms}ms"
           sleep scan_interval.minutes
         end
       end
@@ -89,6 +124,7 @@ class Library
   end
 
   def scan
+    start = Time.local
     unless Dir.exists? @dir
       Logger.info "The library directory #{@dir} does not exist. " \
                   "Attempting to create it"
@@ -97,14 +133,36 @@ class Library
 
     storage = Storage.new auto_close: false
 
+    examine_context : ExamineContext = {
+      cached_contents_signature: {} of String => String,
+      deleted_title_ids:         [] of String,
+      deleted_entry_ids:         [] of String,
+    }
+
+    @title_ids.select! do |title_id|
+      title = @title_hash[title_id]
+      existence = title.examine examine_context
+      unless existence
+        examine_context["deleted_title_ids"].concat [title_id] +
+                                                    title.deep_titles.map &.id
+        examine_context["deleted_entry_ids"].concat title.deep_entries.map &.id
+      end
+      existence
+    end
+    remained_title_dirs = @title_ids.map { |id| title_hash[id].dir }
+    examine_context["deleted_title_ids"].each do |title_id|
+      @title_hash.delete title_id
+    end
+
+    cache = examine_context["cached_contents_signature"]
     (Dir.entries @dir)
       .select { |fn| !fn.starts_with? "." }
       .map { |fn| File.join @dir, fn }
+      .select { |path| !(remained_title_dirs.includes? path) }
       .select { |path| File.directory? path }
-      .map { |path| Title.new path, "" }
+      .map { |path| Title.new path, "", cache }
       .select { |title| !(title.entries.empty? && title.titles.empty?) }
       .sort! { |a, b| a.title <=> b.title }
-      .tap { |_| @title_ids.clear }
       .each do |title|
         @title_hash[title.id] = title
         @title_ids << title.id
@@ -113,8 +171,15 @@ class Library
     storage.bulk_insert_ids
     storage.close
 
-    Logger.debug "Scan completed"
-    Storage.default.mark_unavailable
+    ms = (Time.local - start).total_milliseconds
+    Logger.info "Scanned #{@title_ids.size} titles in #{ms}ms"
+
+    Storage.default.mark_unavailable examine_context["deleted_entry_ids"],
+      examine_context["deleted_title_ids"]
+
+    spawn do
+      save_instance
+    end
   end
 
   def get_continue_reading_entries(username)
diff --git a/src/library/title.cr b/src/library/title.cr
index 957a608..7efee42 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -2,18 +2,24 @@ require "digest"
 require "../archive"
 
 class Title
+  include YAML::Serializable
+
   getter dir : String, parent_id : String, title_ids : Array(String),
     entries : Array(Entry), title : String, id : String,
     encoded_title : String, mtime : Time, signature : UInt64,
     entry_cover_url_cache : Hash(String, String)?
   setter entry_cover_url_cache : Hash(String, String)?
 
+  @[YAML::Field(ignore: true)]
   @entry_display_name_cache : Hash(String, String)?
+  @[YAML::Field(ignore: true)]
   @entry_cover_url_cache : Hash(String, String)?
+  @[YAML::Field(ignore: true)]
   @cached_display_name : String?
+  @[YAML::Field(ignore: true)]
   @cached_cover_url : String?
 
-  def initialize(@dir : String, @parent_id)
+  def initialize(@dir : String, @parent_id, cache = {} of String => String)
     storage = Storage.default
     @signature = Dir.signature dir
     id = storage.get_title_id dir, signature
@@ -26,6 +32,7 @@ class Title
       })
     end
     @id = id
+    @contents_signature = Dir.contents_signature dir, cache
     @title = File.basename dir
     @encoded_title = URI.encode @title
     @title_ids = [] of String
@@ -36,7 +43,7 @@ class Title
       next if fn.starts_with? "."
       path = File.join dir, fn
       if File.directory? path
-        title = Title.new path, @id
+        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
@@ -63,6 +70,106 @@ class Title
     end
   end
 
+  # Utility method used in library rescanning.
+  # - When the title does not exist on the file system anymore, return false
+  #     and let it be deleted from the library instance
+  # - When the title exists, but its contents signature is now different from
+  #     the cache, it means some of its content (nested titles or entries)
+  #     has been added, deleted, or renamed. In this case we update its
+  #     contents signature and instance variables
+  # - When the title exists and its contents signature is still the same, we
+  #     return true so it can be reused without rescanning
+  def examine(context : ExamineContext) : Bool
+    return false unless Dir.exists? @dir
+    contents_signature = Dir.contents_signature @dir,
+      context["cached_contents_signature"]
+    return true if @contents_signature == contents_signature
+
+    @contents_signature = contents_signature
+    @signature = Dir.signature @dir
+    storage = Storage.default
+    id = storage.get_title_id dir, signature
+    if id.nil?
+      id = random_str
+      storage.insert_title_id({
+        path:      dir,
+        id:        id,
+        signature: signature.to_s,
+      })
+    end
+    @id = id
+    @mtime = File.info(@dir).modification_time
+
+    previous_titles_size = @title_ids.size
+    @title_ids.select! do |title_id|
+      title = Library.default.get_title! title_id
+      existence = title.examine context
+      unless existence
+        context["deleted_title_ids"].concat [title_id] +
+                                            title.deep_titles.map &.id
+        context["deleted_entry_ids"].concat title.deep_entries.map &.id
+      end
+      existence
+    end
+    remained_title_dirs = @title_ids.map do |title_id|
+      title = Library.default.get_title! title_id
+      title.dir
+    end
+
+    previous_entries_size = @entries.size
+    @entries.select! do |entry|
+      existence = File.exists? entry.zip_path
+      Fiber.yield
+      context["deleted_entry_ids"] << entry.id unless existence
+      existence
+    end
+    remained_entry_zip_paths = @entries.map &.zip_path
+
+    is_titles_added = false
+    is_entries_added = false
+    Dir.entries(dir).each do |fn|
+      next if fn.starts_with? "."
+      path = File.join dir, fn
+      if File.directory? path
+        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
+        next
+      end
+      if is_supported_file path
+        next if remained_entry_zip_paths.includes? path
+        entry = Entry.new path, self
+        if entry.pages > 0 || entry.err_msg
+          @entries << entry
+          is_entries_added = true
+        end
+      end
+    end
+
+    mtimes = [@mtime]
+    mtimes += @title_ids.map { |e| Library.default.title_hash[e].mtime }
+    mtimes += @entries.map &.mtime
+    @mtime = mtimes.max
+
+    if is_titles_added || previous_titles_size != @title_ids.size
+      @title_ids.sort! do |a, b|
+        compare_numerically Library.default.title_hash[a].title,
+          Library.default.title_hash[b].title
+      end
+    end
+    if is_entries_added || previous_entries_size != @entries.size
+      sorter = ChapterSorter.new @entries.map &.title
+      @entries.sort! do |a, b|
+        sorter.compare a.title, b.title
+      end
+    end
+
+    true
+  end
+
   alias SortContext = NamedTuple(username: String, opt: SortOptions)
 
   def build_json(*, slim = false, shallow = false,
diff --git a/src/library/types.cr b/src/library/types.cr
index 0545184..4c9dc93 100644
--- a/src/library/types.cr
+++ b/src/library/types.cr
@@ -133,3 +133,8 @@ class TitleInfo
     LRUCache.set generate_cache_entry key, self.to_json
   end
 end
+
+alias ExamineContext = NamedTuple(
+  cached_contents_signature: Hash(String, String),
+  deleted_title_ids: Array(String),
+  deleted_entry_ids: Array(String))
diff --git a/src/mango.cr b/src/mango.cr
index f27165e..39b1352 100644
--- a/src/mango.cr
+++ b/src/mango.cr
@@ -58,6 +58,7 @@ class CLI < Clim
       LRUCache.init
       Storage.default
       Queue.default
+      Library.load_instance
       Library.default
       Plugin::Downloader.default
 
diff --git a/src/storage.cr b/src/storage.cr
index 39116b9..32f446a 100644
--- a/src/storage.cr
+++ b/src/storage.cr
@@ -428,12 +428,21 @@ class Storage
     end
   end
 
-  def mark_unavailable
+  # Mark titles and entries that no longer exist on the file system as
+  #   unavailable. By supplying `id_candidates` and `titles_candidates`, it
+  #   only checks the existence of the candidate titles/entries to speed up
+  #   the process.
+  def mark_unavailable(ids_candidates : Array(String)?,
+                       titles_candidates : Array(String)?)
     MainFiber.run do
       get_db do |db|
         # Detect dangling entry IDs
         trash_ids = [] of String
-        db.query "select path, id from ids where unavailable = 0" do |rs|
+        query = "select path, id from ids where unavailable = 0"
+        unless ids_candidates.nil?
+          query += " and id in (#{ids_candidates.join "," { |i| "'#{i}'" }})"
+        end
+        db.query query do |rs|
           rs.each do
             path = rs.read String
             fullpath = Path.new(path).expand(Config.current.library_path).to_s
@@ -449,7 +458,11 @@ class Storage
 
         # Detect dangling title IDs
         trash_titles = [] of String
-        db.query "select path, id from titles where unavailable  = 0" do |rs|
+        query = "select path, id from titles where unavailable = 0"
+        unless titles_candidates.nil?
+          query += " and id in (#{titles_candidates.join "," { |i| "'#{i}'" }})"
+        end
+        db.query query do |rs|
           rs.each do
             path = rs.read String
             fullpath = Path.new(path).expand(Config.current.library_path).to_s
diff --git a/src/util/signature.cr b/src/util/signature.cr
index d1a0040..5ca3e14 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -48,4 +48,32 @@ class Dir
     end
     Digest::CRC32.checksum(signatures.sort.join).to_u64
   end
+
+  # Returns the contents signature of the directory at dirname for checking
+  #   to rescan.
+  # Rescan conditions:
+  #   - When a file added, moved, removed, renamed (including which in nested
+  #       directories)
+  def self.contents_signature(dirname, cache = {} of String => String) : String
+    return cache[dirname] if cache[dirname]?
+    Fiber.yield
+    signatures = [] of String
+    self.open dirname do |dir|
+      dir.entries.sort.each do |fn|
+        next if fn.starts_with? "."
+        path = File.join dirname, fn
+        if File.directory? path
+          signatures << Dir.contents_signature path, cache
+        else
+          # Only add its signature value to `signatures` when it is a
+          #   supported file
+          signatures << fn if is_supported_file fn
+        end
+        Fiber.yield
+      end
+    end
+    hash = Digest::SHA1.hexdigest(signatures.join)
+    cache[dirname] = hash
+    hash
+  end
 end