def get_book_hierarchy_id(self):
<EOL>if self._catalog_session is not None:<EOL><INDENT>return self._catalog_session.get_catalog_hierarchy_id()<EOL><DEDENT>return self._hierarchy_session.get_hierarchy_id()<EOL>
Gets the hierarchy ``Id`` associated with this session.

        return: (osid.id.Id) - the hierarchy ``Id`` associated with this
                session
        *compliance: mandatory -- This method must be implemented.*