Paladin's Hope (The Saint of Steel #3) by T. Kingfisher

Piper is a lich-doctor, a physician who works among the dead, determining causes of death for the city guard's investigations. It's a peaceful, if solitary profession…until the day when he's called to the river to examine the latest in a series of mysterious bodies, mangled by some unknown force.

Galen is a paladin of a dead god, lost to holiness and no longer entirely sane. He has long since given up on any hope of love. But when the two men and a brave gnole constable are drawn into the web of the mysterious killer, it's Galen's job to protect Piper from the traps that await them.

He's just not sure if he can protect Piper from the most dangerous threat of all…