A "locked room mystery" is a fictional type of thriller novel that involves a crime or crimes (usually, murder) that occurs amongst a small group of people that are shut off from the outside world, in a setting like a remote island or snowed-in cabin. Based on the secluded setting, the criminal(s) necessarily must be amongst the small group of people. In other words, it's as if a room were locked with all the people in it prior to the occurrence of a crime taking place amongst them inside the room. Popular authors who often write this type of mystery include Agatha Christie, Ruth Ware, and Lucy Foley -- all of whose books you will find below.