Rules for Perfect Murders

"Specialising in mystery books, Old Devils is a Boston institution; a place where tourists come to snap photos with the shop's Instagrammable cat and browse first editions of Patricia Highsmith novels. Malcolm Kershaw, its owner, lives a quiet life, so he's surprised when an FBI Agent shows up one gloomy winter morning and asks him if he can answer some questions. Agent Gwen Mulvey is investigatin…