'Return the OPF metadata backup for the book as a bytestring or None
if no such backup exists.'

@read_api
def read_backup(self, book_id):

    try:
        path = self._field_for(u'path', book_id).replace(u'/', os.sep)
    except:
        return
    try:
        return self.backend.read_backup(path)
    except EnvironmentError:
        return None
