Nine Perfect Strangers by Liane Moriarty
Read Watch & Wine - A podcast by readwatchwine - Wednesdays
Nine people from different walks of life attend a pricey 10-day "Mind and Body Total Transformation Retreat" at a place called the Tranquillum House run by a mysterious Russian woman named Masha. Throughout the course of their retreat, they realize that each of them is battling their own demons and all of them are subjects of an experiment.