I suppose it can't really count as a spoiler if it's mentioned in a book's first line, never mind that it was revealed on the author's website and (I'm told) his Twitter feed prior to publication, plus it's in the jacket copy, but I still feel like I ought to warn people to brace themselves for this one: Harry Dresden has a daughter.
It comes as a shock to him too. The girl's mother, Susan Rodriguez, exited the series seven books ago after an attack by Red Court vampires began the process of turning her into one of them; she joined the Fellowship of St. Giles to fight both the Red Court and her own half-vampire nature. Now it turns out she hid her child from everyone, including Harry, to protect her, but word of her got back to exactly the wrong vampire, and little Maggie has been kidnapped by Red Court Duchess Arianna Ortega. Harry knows he'll need more power than he and his personal friends can raise to rescue his child, and when every supposedly good entity he goes to for help turns him down, for what seem to them to be insurmountable reasons, more questionable sources of power come inevitably to mind.
Twelve books into the series, it's getting difficult to summarize a new entry without recapping a hell of a lot of backstory. I'm not really complaining about that; one of the things I really like about the series as a whole is the way that seemingly minor characters or plot points in early books later turn out to have much more significance than they appeared to at first. This volume even has a callback to the barely competent sorcerer Harry defeated in the very first book, Storm Front, and includes nearly all of the supporting cast Harry has built up over the whole series, though a number of them (Marcone, Kincaid, and the archangel Uriel among these) only make cameo appearances.
This one is also the first to end on such an appalling cliffhanger. And I have to wait another year to find out what that last page meant? I'm also really looking forward to exploring the ramifications of some of the hard choices Harry had to make this time, and I'm just glad Jim Butcher has no record of missing his deadlines.