Question: who wrote the first book about king arthur
[{'answer': 'Historia Regum Britanniae', 'provenance': [{'wikipedia_id': '16808', 'title': 'King Arthur', 'start_paragraph_id': 2, 'start_character': 240, 'end_paragraph_id': 2, 'end_character': 265, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '16808', 'title': 'King Arthur', 'start_paragraph_id': 25, 'start_character': 0, 'end_paragraph_id': 25, 'end_character': 1795, 'bleu_score': 0.8804895155685191, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13469888446270611445}, 'section': 'Section::::Medieval literary traditions.:Geoffrey of Monmouth.\n'}]}]