@abc.abstractmethod<EOL><INDENT>def get_child_books(self, book_id):<DEDENT>
return<EOL>
Gets the child books of the given ``id``.

        :param book_id: the ``Id`` of the ``Book`` to query
        :type book_id: ``osid.id.Id``
        :return: the child books of the ``id``
        :rtype: ``osid.commenting.BookList``
        :raise: ``NotFound`` -- a ``Book`` identified by ``Id is`` not found
        :raise: ``NullArgument`` -- ``book_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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