Mind's Eye — Hakan Nesser

The swift conviction left Van Veeteren uneasy: Janek Mitter woke one morning with a brutal hangover and his wife dead in the bathtub. With only the flimsiest defense, he is found guilty and committed to a mental institution. But when Mitter is murdered in his cell, Van Veeteren regrets not following his gut and launches an investigation into the two killings. As the chief inspector delves deeper, the twisted root of these violent murders will shock even him.