@abc.abstractmethod<EOL><INDENT>def get_root_books(self):<DEDENT>
return<EOL>
Gets the root books in the book hierarchy.

        A node with no parents is an orphan. While all book ``Ids`` are
        known to the hierarchy, an orphan does not appear in the
        hierarchy unless explicitly added as a root node or child of
        another node.

        :return: the root books
        :rtype: ``osid.commenting.BookList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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