The Housemaid by Freida McFadden

A live-in maid discovers the door to her attic room locks from the outside, but her employers have no idea how dangerous she really is.

Millie Calloway is desperate for a job, so she doesn't ask questions when the wealthy Winchesters hire her as their live-in maid. But life in their mansion isn't the fresh start she imagined. Nina Winchester is erratic and cruel, while her husband, Andrew, seems like a man trapped in a broken marriage. As Millie settles into her room in the attic—the door of which only locks from the outside—she begins to realize the family’s secrets are darker than her own. The Housemaid is a high-stakes psychological thriller where every character is hiding a past, and the truth is far more dangerous than the lie.