Paladin's Faith (Saint of Steel‪)‬

From Hugo Award winning author T. Kingfisher:

Marguerite Florian is a spy with two problems. A former employer wants her dead, and one of her new bodyguards is a far too good-looking paladin with a martyr complex.

Shane is a paladin with three problems. His god is dead, his client is much too attractive for his peace of mind, and a powerful organization is trying to have them both killed.

Add in a brilliant artificer with a device that may change the world, a glittering and dangerous court, and a demon-led cult, and Shane and Marguerite will be lucky to escape with their souls intact, never mind their hearts . . .