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