document.rb

module Eader
  class Document
    attr_reader :doc

    def initialize(path)
      file = open(path)
      @doc = xml_doc_class.parse(file)
    end

    def series
      doc.css('dsc c01').map do |c|
        if c['level'] == 'series'
          Item.new(c)
        end
      end.compact
    end
    
    #...
  end
end