I've tried to make a point of reviewing each of these novels as I've re-read them, but frankly, I feel like I'm getting repetitive. The novels are almost formulaic (a complaint Butcher himself made even as he started writing the series.) That doesn't actually make them any less fun... but it means that it's harder to find something unique to say about them as I go further into the series. Protagonist Harry Dresden finds himself put out more each and every volume. A friend of mine complained recently about how mad it made her to see the stuff Butcher puts Dresden through. She hadn't read very far into the series, though... much worse is still to come. In Death Masks the toll Dresden takes is more emotional than anything else, but it's significant.
As with all the Dresden books, part of the fun is the unraveling of setting stuff that he continues to do. For this installment, we find out that Michael Carpenter isn't a unique character exactly; he belongs to an order of two other Knights of the Cross. All three of them spend a fair amount of time in this book, working with Harry against the Denarians, fallen angels associated with the thirty pieces of silver paid to Judas for betraying Christ. The fallen angels tempt and then possess mortals, effectively becoming embodied demons with staggering supernatural power and staggering supernatural evil. Their leader, Nicodemus, is the most human-like of them, but then again he's the most evil. So evil, in fact, that he's not possessed in the traditional sense; he's a willing partner with the fallen angel. There's a plot involving the shroud of Turin, which has been stolen and which Dresden is hired to recover.
In addition, the war between the Red Court of the Vampires and the White Council of the wizards (not to be confused with the White Court of Vampires, which features heavily in the next book) ramps up and a warlord, Duke Ortega, of the vampires, comes to Chicago to kill Harry.
All in all a fun book, and like all of them, the setting is as much a character as anything else. The Carpenters are further defined. Harry's difficult relationship with estranged erstwhile flame Susan Rodriguez is dealth with in depth. Ebeneezer McCoy starts to move into the orbit as a "more significant character" who gets more screentime.
As always, I recommend it. It took me a little while to get started, because I was really busy, but as soon as I really got going, I devoured it in just a few hours and started immediately on the next one. It got me that excited.