'Return all the books associated with the item identified by
``item_id``, where the item belongs to the field ``name``.
Returned value is a set of book ids, or the empty set if the item
or the field does not exist.'

@read_api
def books_for_field(self, name, item_id):

    try:
        return self.fields[name].books_for(item_id)
    except (KeyError, IndexError):
        return set()
