详情参见：http://about.ac/2015/04/duokanify.html

Rakefile

require 'fileutils'

# Usage: rake duokanify file=path/to/sample.epub
desc 'Adds dtb:depth to ncx file in an ePub'
task :duokanify do
  unless ENV['file']
    abort("ERROR: Please provide the ePub file path!\nexit")
  end

  unless ENV['file'].end_with?('.epub')
    abort("ERROR: The file you provided is not an ePub!\nexit")
  end

  path = File.expand_path(ENV['file'])

  unless File.exist?(path)
    abort("ERROR: The file you provided is not exists!\nexit")
  end

  puts 'Unzipping....'
  dest_dir = File.basename(path, '.epub')
  system "unzip #{ENV['file']} -d #{dest_dir}"

  puts
  puts 'Normalize....'
  ncx_file = File.join(dest_dir, 'OEBPS', 'toc.ncx')
  unless File.exist?(ncx_file)
    abort("Info: The ePub has no ncx file, needless to normalize.\nexit")
  end
  ncx = File.open(ncx_file, 'r+') do |f|
    content = f.read
    # puts content.inspect
    if content =~ /meta="dtb:depth"/i
      abort("Info: The ePub already set dtb:depth, needless to normalize.\nexit")
    else
      parts = content.split('</head>')
      parts.first << '<meta name="dtb:depth" content="1"/>'
      f.rewind
      f.write parts.join('</head>')
    end
  end

  puts
  puts 'Rezipping....'
  normalized_file = "#{File.basename(dest_dir)}-normalized.epub"
  FileUtils.cd(dest_dir) do
    system "zip -0 -X #{normalized_file} mimetype"
    system "zip -9 -r #{normalized_file} META-INF OEBPS"
  end
  FileUtils.cp File.join(dest_dir, normalized_file), Dir.pwd

  puts
  puts 'Cleanning....'
  FileUtils.rm_rf(dest_dir, :secure => true)

  puts
  puts 'Done'
  puts "Normalized ePub path: ./#{normalized_file}"
end