I've already discussed at length my reservations about the series overall, so I won't rehash that here. Rather, I'll focus on the specifics of this novel.
I quite like it, as I like all of the Dresden Files books, especially the second and beyond. However, it wasn't the strongest of them. There's a lot of good stuff around character development. Harry Dresden himself is put through a real wringer, and emerges a much richer character. He undergoes a number of really life-defining moments in this book. His girlfriend, Susan Rodriguez, does as well.
Also, this book introduces recurring character Michael, the Fist of God. Michael is the wielder of Amoracchius (always written in italics) which the books hint might be the actual real sword with is the legend of Excalibur. Michael isn't a magician in the strictest sense of any kind, but as a highly favored of the Lord righteous man with a magic sword, his "faith magic" is actually quite powerful. Butcher took a slightly more difficult road here, and I applaud him for it. Given his probable audience, it would have been easy for him to cast the Catholic Church and anyone associated with it in a bad light. Dresden himself states that although he believes in God, "he doesn't exactly see eye to eye with him." However, in contrast to that, we get repeated exposure to the character of Michael, which is everything that a member of the Church should be and hopefully aspires to. He's a great character, and no less interesting for his squeeky clean image. Anyway, Michael recurs frequently throughout the series, and his oldest daughter actually ends up becoming Harry's apprentice later on, so the impact of Michael (and his family) is substantial. I'd say other than Karrin Murphy and possibly Thomas Raith, he's the most important non-protagonist character to appear in the series. Thomas Raith is also introduced in this novel, by the way, although his future importance is barely hinted at. Murphy, on the other hand, gets very little screentime.
Anyway, the line-up of characters who appear, and what they will do later sounds more fanboyish than helpful, so I will point out that plot-wise, this one probably has a little too much going on, frankly. Who's the bad guys here? The ghosts? The vampires? The sidhe? All of the above? The lack of focus means that they aren't really given their due. Butcher instead focused on setting up important plot arcs that would take him through much of the rest of the series, and character development. While this isn't exactly problematic, in some of the other books (Fool Moon for example, which came right before this) the horrific nature of the "monster of the year" is more fully explored, and creates a much more dramatic effect.
I suppose that's the challenge of writing a series of mostly self-contained, yet loosely connected novels; the self-contained aspect tends to create friction against the connected aspect. In this case, the connections with other novels (future novels, mostly) are stronger while the self-contained aspect is a bit weaker. That makes the novel itself a bit weaker, but the series itself stronger.
But, I'm really not trying to gripe. It's not that big a deal; the novel is still plenty of fun, the bad guys are still scary enough, the characters are still charming, and the plot is still engaging. Everything that makes a Dresden Files novel fun to read is still here, and there's plenty of it.