Breaking Out the Devil — Jane Yolen; Adam Stemple

The blazing conclusion to the Stone Man Mysteries trilogy.
In the climactic final volume of the Stone Man Mysteries series, the demon-turned-gargoyle Silex and his human helper Craig make the ultimate uneasy alliance: a partnership with the Devil himself. The prince of darkness is stuck on Earth, trapped in the body of Silex's previous assistant. When one of the Devil's lieutenants attempts to conquer both the underworld and living world, Silex and Craig will have to bust the Devil out of confinement in order to save humanity.