code: def add_book_series(self, title, volume=None):
          
          book_series = {}
          if title is not None:
              book_series['title'] = title
          if volume is not None:
              book_series['volume'] = volume
  
          self._append_to('book_series', book_series)
docs: :param volume: the volume of the book
          :type volume: string
  
          :param title: the title of the book
          :type title: string