In the House of the Wicked by Thomas Sniegoski

 This is possibly the darkest book in the series yet. On the surface, things are going well. Remy has gotten past the grief for his ex-wife enough to begin dating a nice woman, Linda, finally, and he has nearly integrated his dual nature as a seraphim once more, accepting the power when it is needed to right wrongs, but not letting the vengeful, warrior nature overcome his human niceness.

The group of fallen angels called the Grigori, whom we've encountered before, have joined forces with the most powerful individual in a cabal of sorcerers in a plot which endangers millions of people around the world, and another powerful sorcerer kidnaps Remy's neighbor girl, Ashley, who watches Marley when he has to go out of town. When Remy journeys to the shadowland where Ashley is being held captive, the magician and his golem minions drain the angel of nearly all of his seraphic powers, and leave him weakened to nearly the level of a normal human. Despite this handicap, Remy presses on trying to save the girl and the world.