How many books are in the Percy Jackson series?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q30', 'id': 0, 'label': 'United States of America'}, {'type': 'INTERNAL', 'name_': 'Q19219752', 'id': 1, 'label': 'Meyers Konversations-Lexikon, 4th edition (1885–1890)'}, {'type': 'INTERNAL', 'name_': 'Q41680', 'id': 2, 'label': 'Dionysus'}, {'type': 'INTERNAL', 'name_': 'Q456794', 'id': 3, 'label': 'Rufus King'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q23806', 'id': 4, 'label': '10'}, {'type': 'INTERNAL', 'name_': 'Q901384', 'id': 5, 'label': 'The Sea of Monsters'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q3899725', 'id': 6, 'label': 'Percy Jackson'}, {'type': 'INTERNAL', 'name_': 'Q97841736', 'id': 7, 'label': 'The Tower of Nero'}], 'links': [{'name_': 'P17', 'source': 0, 'target': 0, 'label': 'country'}, {'name_': 'P1343', 'source': 2, 'target': 1, 'label': 'described by source'}, {'name_': 'P27', 'source': 3, 'target': 0, 'label': 'country of citizenship'}, {'name_': 'P166', 'source': 3, 'target': 4, 'label': 'award received'}, {'name_': 'P1343', 'source': 4, 'target': 1, 'label': 'described by source'}, {'name_': 'P495', 'source': 5, 'target': 0, 'label': 'country of origin'}, {'name_': 'P674', 'source': 5, 'target': 6, 'label': 'characters'}, {'name_': 'P674', 'source': 7, 'target': 2, 'label': 'characters'}, {'name_': 'P674', 'source': 7, 'target': 6, 'label': 'characters'}]}