Witch Elm by Tana French

Three cousins grow up together hanging out with their favorite Uncle Hugo in his rambling home and garden. Now years later,one of the cousins has been severely beaten and they learn that Uncle Hugo is dying. To ease Toby's recovery and help care for Hugo, Toby and his girlfriend move into Hugo's house. What begins as a sweet reunion and reminiscing quickly escalates into a police matter as human remains are discovered in the witch elm tree in Hugo's garden and someone who used to hang out at the house must be responsible for the death. Once the remains are identified as a classmate of the cousins who bullied all of them it is easy to see why he was killed but not who did it. An intense murder mystery and clever story of friendship, family loyalty, and family secrets.