Paladin's Grace (The Saint of Steel, #1) by T. Kingfisher

Stephen's god died on the longest day of the year...

Three years later, Stephen is a broken paladin, living only for the chance to be useful before he dies. But all that changes when he encounters a fugitive named Grace in an alley and witnesses an assassination attempt gone wrong. Now the pair must navigate a web of treachery, beset on all sides by spies and poisoners, while a cryptic killer stalks one step behind...

From the Hugo and Nebula Award winning author of Swordheart and The Twisted Ones comes a saga of murder, magic, and love on the far side of despair.