The Witch Elm by Tana French

Toby Hennessy lives a charmed life. He's wealthy with a great girlfriend, Melissa, and everything has always gone his way. But one evening he's attacked in his home by two men and suffers a brain injury. He has a hard time remembering things from the past and also physically isn't the same. Not able to go back to work, Toby decides to go live at Ivy House, the Hennessy family house for many generations--now occupied by Toby's Uncle Hugo, who dying from cancer, with only months to live. Since Toby and his cousins, Susanna and Leon, have spent a lot of time with Hugo since they were very young, Toby figures it would be a good diversion from his own issues. When a skull is found in an old tree on the Ivy House property, Toby's life is thrown even more into disarray...