Deadly Communion (aka Vienna Twilight) (Liebermann Papers, #05)

Detective Inspector Oskar Reinhardt finds that young women are being slain in an unnerving—and ingenious—manner, with a small, almost undetectable, hat pin. For Dr. Max Liebermann, the killer is unique in the annals of psychopathology, one who murders in the midst of consensual love. Is the culprit a patient, one who swears he has a double, a shadow figure that is far more forward (in fact, indecent) with women? As danger mounts, Liebermann must find the answer while struggling with his own forbidden desire for a former female patient.