City of the Sun — David Levien

Private detective Frank Behr has been content living a solitary life, working on a few simple cases, and attempting to move on from his painful past. But when Paul and Carol Gabriel ask him to help them find their missing son, he can hardly refuse. Going against everything he fears -- Behr's been around too long to hope for a happy ending -- he enters into an uneasy partnership with Paul on a quest for the truth that
will become both dangerous and haunting.