docs: def get_books(self):
          """Gets the book list resulting from a search.
  
          return: (osid.commenting.BookList) - the book list
          raise:  IllegalState - list has already been retrieved
          *compliance: mandatory -- This method must be implemented.*
  
          """
          if self.retrieved:
              raise errors.IllegalState('List has already been retrieved.')
          self.retrieved = True
          return objects.BookList(self._results, runtime=self._runtime)