The Gesture, by Gil Brewer (epub/Kindle/pdf)
Nolan has retired with his wife to a private island off the mainland. He did so because of lifelong jealousy—he can't stand the thought of other men being attracted to Helen. When a photographer from New York shows up one day to do a photo story about the island. Nolan becomes convinced that he is having an affair with Helen—and begins to plan a murder...