根据book_idx计算开始和结束的chapter id
        :param book_idx:
        :type int:
        :return:
        :rtype:

def _start_end_of_index(self, book_idx):
        """
        根据book_idx计算开始和结束的chapter id
        :param book_idx:
        :type int:
        :return:
        :rtype:
        """
        start = (book_idx - 1) * self._max_chapters
        end = min(book_idx * self._max_chapters, len(self._chapters))
        return (start, end)