Truly Darkly Deeply by Victoria Selman

After moving to London, twelve-year-old Sophie and her mother, Amelia-Rose, meet the charismatic Matty Melgren, who quickly becomes an intrinsic part of their lives. Sophie adores Matty: he's handsome, funny and respectable. But just as the relationship between the two adults fractures, a serial killer begins targeting women with a striking resemblance to Amelia Rose-and the police sketch of the suspect looks an awful lot like Matty....

Sophie, consumed with doubt, acts impulsively, tearing her family apart. After Matty is arrested, convicted and sentenced to life without parole, haunting questions remain as to his guilt. Years later, Sophie receives a letter from Matty, now dying, asking her to visit him in prison. She might finally get answers she's been searching for. But will the truth set Sophie free or bury her deeper.