John D. MacDonald 
Where is Janice Gantry? 

Sam Brice, the perfect rugged hero, is involved with a young widow named Janice Gantry. But when she vanishes, Janice leaves behind a trail of blackmail, murder and a man at war with his own sense of duty.Sam is too curious to steer clear of the mystifying disappearances off the Florida Keys, too stubborn to avoid making enemies with a cunning criminal and killer, and perhaps too enraged to do what he knows he must to save the one woman who matters: become as cold, impersonal, and deadly as an assassin.