From https://once.com/writebook

app/controllers/books/bookmarks_controller.rb

@@ -0,0 +1,14 @@
+class Books::BookmarksController < ApplicationController
+  allow_unauthenticated_access
+
+  include BookScoped
+
+  def show
+    @leaf = @book.leaves.active.find_by(id: last_read_leaf_id) if last_read_leaf_id.present?
+  end
+
+  private
+    def last_read_leaf_id
+      cookies["reading_progress_#{@book.id}"]
+    end
+end