def add_book_series(self, title, volume=None):
        """
        :param volume: the volume of the book
        :type volume: string

        :param title: the title of the book
        :type title: string
        """
        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)