@abc.abstractmethod<EOL><INDENT>def get_comments_by_books(self, book_ids):<DEDENT>
return<EOL>
Gets the list of ``Comments`` corresponding to a list of ``Books``.

        :param book_ids: list of book ``Ids``
        :type book_ids: ``osid.id.IdList``
        :return: list of comments
        :rtype: ``osid.commenting.CommentList``
        :raise: ``NullArgument`` -- ``book_ids`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*