The Sirens - The Midnight Eye Files, #2

Glasgow private investigator Derek Adams can't seem to get away from the otherworldly, especially not when it seems to be the only paying gig in town. So when an old widow offers him two grand to find her son-who isn't exactly missing...just not himself-it's not long before things start to get weird, and a road trip to the rural village of Skye turns into a detour to the twilight. Before long, Derek is on a remote island, hip-deep in mermaids, stalked by shape-shifters, and tangled in the nets of a diabolical fisher cult intent on waking an ancient god and unleashing the apocalypse.
In other words, it's business as usual.