Online reviews for “Nature of the Beast” by Louise Penny are split. Many readers found the story too far-fetched. A “supergun,” found decades after the end of the Cold War, in a remote Canadian forest? Discovered only after the nearby murder of a troubled young boy? Whatta stretch. Even more of a stretch: The weapon is found near the tiny village of Three Pines, where Chief Inspector Armand Gamache and his wife, Reine-Marie, have retired.