Where Is Janice Gantry? — John D. MacDonald

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.