The Family Upstairs by Lisa Jewell

Libby Jones has long known that when she turns 25, she inherits something from her birth parents' trust, but she is stunned to discover that it turns out a be a decaying mansion in one of the most expensive areas of London. It also is where her birth parents, Henry and Martina Lamb, died in a cult-like situation when she was ten months old. The Family Upstairs recounts Libby's journey of discovering what happened in the family home all those years ago, interspersed with Henry Jr's retelling of the years when his family's status went from wealth and privilege to being prisoners in their own home. In the novel, the reader also meets Lucy, who lives a meager existence in France with her two children, but longs to get back to Britain, now that the baby (aka Libby) is now 25. The Family Upstairs is a creepily unputdownable read. Perfect for those who love Ruth Rendell's psychological novels, Lying in Wait by Liz Nugent, and the books of Erin Kelly.