Jim Butcher introduced the idea of holy knights a few books ago, but the concept didn't fully flower until the fifth Dresden Files book, "Death Masks." It's a big dark mass of devils, divine knights and the occasional nefarious vampire -- and while one part of the story is painfully contrived, it has plenty of meaty plot mixed in there. 
 Harry is hired by a Vatican priest named Father Vincent, who wants him to recover a famous Catholic relic -- the Shroud of Turin -- which has been stolen and brought to Chicago by parties unknown. Unfortunately, a special group of Fallen angels called the Denarians are also pursuing the Shroud, because it can help them unleash a superplague that could kill the entire human race. 
 In the meantime, Harry has also been challenged to a death duel by Ortega, a vampire noble who wants to somehow stop the war. Also, his ex-girlfriend-turned-demi-vampire Susan has returned to Chicago, stirring up some old romantic feelings. As the forces of evil prepare to wipe out the human race, Harry may be the one person who can save us all... but not without some losses. 
 Jim Butcher has introduced three breeds of vampires, several kinds of werewolves, and two vast courts of faeries, and "Death Masks" adds Judeo-Christian angels and demons (or "The Fallen") to his world. This book introduces a lot of Roman Catholic beliefs and cosmology into the story... but don't worry, it's not preachy or anything of the sort. 
 No, the big draw here is HARRY DRESDEN FIGHTS DEVILS, which are truly grotesque and terrifying -- especially since they all take human form in a corrupted, malevolent human body. Butcher weaves together a bunch of strong subplots, all with a sharp dark edge as well as a touch of humor. "My faith protects me. My Kevlar helps," Michael explains in one scene, confirming why I like this particular saintly character. 
 The biggest problem is that the plot hinges on a really, REALLY contrived plot point. It only works because the Vatican JUST HAPPENED to send a priest who JUST HAPPENED to know another priest that Harry happens to be friends with. Um. 
 But Butcher doesn't fail us in the characterization. He introduces some fun new characters like the adorably buttkicking Shiro, Sanya (an atheist working for God) and the little-girly repository "Ivy," and fleshes out older ones like the cold mobster Marcone. Harry himself has become a stronger man who can now cope with his guilt over Susan, but he's also taken a darker turn (in one scene he beats a Denarian with a baseball bat). 
 Iif you can overlook that one huge contrivance, "Death Masks" a powerful and complex urban fantasy, with plenty of grotesque demons, vampires, and the occasional floating blob of instant doom.