The Dying Trade — Peter Corris

Hardy needs work. In fact, he's the type of detective who never turns down a case. He can afford not to. So when wealthy Bryn Gutteridge, a real estate heir who amuses himself by shooting seagulls, asks Hardy to find out who has been threatening his twin sister, Susan, the private eye agrees. And finds himself on a case that turns brutal everyday. First Gutteridge's butler is murdered. Then his pretty young stepmother is badly beaten. Hardy himself takes a few punches. And before long it's hard to tell the victims from the villains.