McNally's Risk by Lawrence Sanders
It was a typical Palm Beach love triangle. He was rich, she was beautiful and his mother just wanted to be absolutely sure that she loved him more than his trust fund. So she hired Archy McNally, the most urbane and witty of private investigators, to look over her future daughter-in-law.Archy thought that Theodosia Johnson was as pure as she was pretty. Until a fashionable portrait painter died, then a topless dancer - and violence exploded among the languid mansions of Florida's millionaires.