The Secret Keeper by Kate Morton

During a summer party at the family farm in the English countryside, sixteen-year-old Laurel Nicolson has escaped to her childhood tree house and is happily dreaming of the future. She spies a stranger coming up the long road to the farm and watches as her mother speaks to him. Before the afternoon is over, Laurel will witness a shocking crime. A crime that challenges everything she knows about her family and especially her mother, Dorothy—her vivacious, loving, nearly perfect mother.