Which is the longest book series?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q30', 'id': 0, 'label': 'United States of America'}, {'type': 'INTERNAL', 'name_': 'Q17', 'id': 1, 'label': 'Japan'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q277759', 'id': 2, 'label': 'book series'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q209026', 'id': 3, 'label': 'Warhammer 40,000'}, {'type': 'INTERNAL', 'name_': 'Q4999841', 'id': 4, 'label': 'Burning Empires'}], 'links': [{'name_': 'P17', 'source': 0, 'target': 0, 'label': 'country'}, {'name_': 'P530', 'source': 0, 'target': 1, 'label': 'diplomatic relation'}, {'name_': 'P1963', 'source': 2, 'target': 1, 'label': 'properties for this type'}, {'name_': 'P495', 'source': 4, 'target': 0, 'label': 'country of origin'}, {'name_': 'P2860', 'source': 4, 'target': 3, 'label': 'cites work'}]}