The fifth book in the Dresden series has Harry fighting the powers of Hell, in not only a battle for the fate of Chicago, his own hide (as usual), the fate of everyone he cares about (a bit new this time) but also the foreshadowings of one of the more interesting internal conflicts that he has to deal with. One of the characters of the book, as a price for their aid, forces Harry to ask himself WHY it is that he does what he does, and while he has to think about it, it is the ultimate answer that helps define the character.
The larger question of the book, what do we do with power when we have it, doesn't seem that prominent compared to the usual amounts of massive action, wanton destruction and extrmely clever lines of dialog. However beneat that, there are larger themes going on, some of which become more appearant reading the books for a second time. Thus, in addition to being Urban Fantasy, the Dresden books manage to be Noir as well; shades of gray with the implication that there also still things that are all black and all white. Grey means a bit more when it really can be gray and it is known that there are extremes.
In some ways, Death Masks feels like a transition between the first few books and the later books, though of course that's a bit like saying link 5 instead of link 6 in a chain is a major transition point. Still, I liked it a great deal.