Scores of books and films have offered explanations, from the plausible to the bizarre, of how an inconsequential person such as Lee Harvey Oswald could have killed the most powerful man in the world. Yet three authors – Larry J. Sabato, a political-science professor at the University of Virginia; Thurston Clarke, a historian; and Philip Shenon, a former New York Times reporter – demonstrate in a trio of new offerings that gifted writers can keep to filling in remaining gaps.