Cactus Island, A Stan Turner Mystery Book Cover

Book description:
Stan is called out to Possum Kingdom Lake in central Texas where two boy scouts have driven off a cliff in a jeep. The driver, Steven Caldwell, survives unscathed but the passenger dies. It appears to be just another tragic auto accident until the sheriff discovers Steven and the victim were in love with the same girl. When Steven is charged with murder, he asks Stan to defend him but Stan is reluctant to do so because Steven claims he ran off the road when he saw a spacecraft hovering overhead. In order to prove Steven's innocence he'd have to prove that aliens had visited Possum Kingdom Lake!