Lars Kepler

"Late one night, outside Stockholm, an emaciated young man named Mikael 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, currently 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....