Dead Beat is the one I just finished. This is one of my favorite books. Not just in this series, but in general. Jim Butcher really pulled out all the stops, and went just enough over the top to be awesome, yet not so far as to get me to roll my eyes. A handful of necromancer and rivals, the last apprentices of the arch-necromancer Heinrich Kemmler (a name that Butcher shamelessly borrows from the Warhammer world) are in town looking for the mysterious Word of Kemmler, a book with the master's final treatise on necromancy, and a rite that gives the user nearly god-like power. Mavra, the Black Court vampire from prior books, shows up blackmailing Dresden to get it to her. Harry's pal Waldo Butters from the coroner's office is targeted by the necromancers, and ends up under Harry's protection.
In another confluence of plot threads, the Denarians make an appearance, notably Lasciel, who's coin Harry picked up a book or two ago, and Quintus Cassius, the one from whom Harry, Sanya and Michael stripped his coin.
Necromancers have always been one of my favorite supernatural bad-guys. There's something that's just plain wrong and creepy about the undead, which is why most horror folklore boils down to ghost stories of one stripe or another. Butcher's always been pretty good about wringing a disturbing aura out of his books, in spite of the sarcastic tone that they're narrated in. In another book, he makes the Billy Goats Gruff really scary, for instance. But it's easy with necromancers, fallen angels, and the worst kinds of monsters that humankind has managed to come up with, all of which feature strongly in Dead Beat.
In addition, much of the action takes place near the Field Museum of Natural History in Chicago. A permanent display at the Field Museum is FMNH PR 2081, the technical designation of "Sue", the largest and most complete Tyrannosaurus rex skeleton unearthed (so far.) And there's necromancers. You do the math.
The interesting thing about this book, and many of the Dresden Files books in particular, is the balance that Butcher strikes between being "episodic" and "serial." That is, is the Dresden Files really a single story that needs to be read in order, or is each episode a separate story that stands completely on its own? Well, none of the books, except the first one, stand completely on their own, and all of them make some references to stuff that happened in prior books. However, Butcher does ensure that you can catch up quickly; if you picked up any Dresden Files book, you'd do OK. Dead Beat, perhaps more than most, draws on several of the open plot threads, refers to past characters (Murphy, Mavra, and even Thomas make little more than cameos, really, for instance), and feels more integrated into the series as a whole. Maybe that's another reason why it's one of my favorites.