The Sandman (Joona Linna #4 - Lars Kepler

Late one night, outside Stockholm, Mikael Kohler-Frost is found wandering. Thirteen years earlier, he went missing along with his younger sister. They were long thought to have been victims of Sweden’s most notorious serial killer, Jurek Walter, now serving a life sentence in a maximum security psychiatric hospital. Now Mikael tells the police that his sister is still alive and being held by someone he knows only as the Sandman.