Over the Edge (Alex Delaware #3) by Jonathan Kellerman

When the phone rings in the middle of the night, child psychologist Alex Delaware does not hesitate. Driving through the dream-lit San Fernando Valley, Alex rushes to Jamey Cadmus, the patient he had failed five years before--and who now calls with a bizarre cry for help.

But by the time Alex reaches Canyon Oaks Psychiatric Hospital, Jamey is gone, surfacing a day later in the hands of the police, who believe Jamey is the infamous Lavender Slasher, a psychotic serial killer. Wooed by a high-powered attorney to build a defense, Alex will get a chance to do what he couldn't five years ago.