Small Favor is the tenth novel of the Dresden Files, following Harry Dresden as he has his second encounter with his arch-nemesis, Nicodemus, from Death Masks. It also follows up on the "three favors for Mab." A plot point which was introduced in Summer Knight and would continue to have a major impact on the series up until Changes. Given the book focuses on the Denarians, it also has a big role for the Knights of the Cross, as well as Gentleman Johnny Marcone a.k.a Chicago's biggest crime-boss and the second most important human who knows about the supernatural in the series.
Harry is recruited by Queen Mab to protect Marcone who is under attacks by a foe who turns out to be the aforementioned Nicodemus. There is much focus on the Unseelie Accords, those nebulous documents created by Queen Mab which are designed to limit the amount of chaos supernatural beings can cause in the modern world. The Knights of the Blackened Denarius, composed of Hell-serving fallen angels, are signatories but don't think much of the rules.
Small Favor is a book which suffers for the fact it is heavily-reliant on continuity. While this is a feature, rather than a bug, for those familiar with series, it means this book would be less enjoyable for a starting volume. Likewise, I'll be honest, Nicodemus is not that great of a villain.
While only the second time he's used, the character doesn't really have that much over Harry's other foes and his returns will become dreaded rather than enjoyed. I really think this should have been his last appearance in the franchise as there is a great moment in this somewhat unremarkable entry in this series which would have been a fitting finale for the demonically possessed mortal.
One thing I dislike about Nicodemus' portrayal in this book is that a character who was initially defined as one of the most intelligent men in history, one of the greatest villains the world has ever seen, and a reader of the Evil Overlord List suffers a great deal of "Badass Decay" (thank you TVtropes.org). Harry and company run rings around him and there's even a moment which lampshades the wizard has a much more formidable foe facing him in the Black Council.
The character is further hurt by the fact he is as close to a loyal servant of evil, which is a questionable motive at the best of times, as the series possesses. To be a good recurring nemesis, a villain must be cool on some level, and it's hard to think of Nicodemus as such when he tortures a twelve-year-old kid as in this book.
In short, Nicodemus starts to wear out his welcome in this book.
Of course, the book does benefit from more focus on the Knights of the Cross as well as series-favorite the Archive. I've always liked the Archive and really wish she'd gotten a bit more focus here. The sum-total repository of all written knowledge in the form of a twelve-year-old girl, Ivy has a spectacularly crappy life. Only Harry Dresden is willing to treat her as a little girl. The book puts her through a truly horrific experience and I'm not sure I'm entirely happy with it.
While menacing a child is certainly a huge motivation for Harry, it's also uncomfortable to read about, and I lost a lot of respect for Nicodemus as an antagonist because of it. Sansa, my favorite of the Knights, also makes a reappearance. More Sansa is always appreciated. I mean, how can you not love a African-Russian atheist-communist Knight of the Christian God? There's also some fun stuff relating to the Three Billy Goats Gruff, except said goats are fairies with guns. No, seriously. Their plotline's resolution is a highlight of the book.
In conclusion, Small Favor is not my favorite of the Dresden Files novels. The return of Nicodemus is underwhelming, there's a bit too much reliance on previous continuity, and the torture poor Ivy undergoes left a sour taste in my mouth. It's not a bad book, though, but suffers in comparison to the great ones which preceded it. Still, I'd rather read Small Favor than most urban fantasy.