Nine Perfect Strangers by Liane Moriarty

The retreat at health-and-wellness resort Tranquillum House promises total transformation. Nine stressed city dwellers are keen to drop their literal and mental baggage, and absorb the meditative ambience while enjoying their hot stone massages. Miles from anywhere, without cars or phones, they have no way to reach the outside world. Just time to think about themselves, and get to know each other. Watching over them is the resort’s director, a woman on a mission. But quite a different one from any the guests might have imagined. For behind the retreat’s glamorous facade lies a dark agenda. These nine perfect strangers have no idea what’s about to hit them . . .