'Return an ordered list of all books in the series that the book identified by index belongs to.
If the book does not belong to a series return an empty list. The list contains book ids.'

def books_in_series_of(self, index, index_is_id=False):

    series_id = self.series_id(index, index_is_id=index_is_id)
    return self.books_in_series(series_id)
