THE MIDNIGHT EYE FILES: THE SIRENS by WILLIAM MEIKLE
The Midnight Eye returns...
At first it's a simple lost son case, but for PI Derek Adams things turn quickly to the twilight zone. Soon he's on a remote island and up to his hips in mer-women, shape-changers and ancient fisher cults.
Running back to the city doesn't help; there's magic and mayhem loose on the streets. An ancient god is waking up, and only blood will stop him!