The Wife Upstairs by Rachel Hawkins

Jane is a dogwalker for a neighborhood of exclusive estates in Alabama. She imagines what it would be like to live in a house like these someday, but never imagines it would actually happen until she meets Eddie Rochester. They begin a whirlwind romance, yet Jane feels like an outsider with the rich ladies of the neighborhood. Eddie had lost his wife Bea and her best friend Blanche in a tragic accident the year before. Bea was the founder of Southern Manors, a sophisticated and cultured lady that everyone strove to be like, while Jane is just the hired help. After Eddie proposes and Jane moves in, rumors begin to fly about the night that the B's disappeared. Jane becomes wary of Eddie, especially when she begins hearing strange thumping noises from upstairs.
I loved this twisty thriller and I didn't guess what was actually happening until the very end. I couldn't put this book down!