Faeries are even better than vampires. Firstly, you can actually make a deal with faeries and compel them to honour the deal. Secondly, that makes them even more deadly, because they're usually clever enough to twist the deal so it ends up harming you anyway. Just as Jim Butcher can't claim credit for vampires, he can't claim credit for faeries, but he sure can claim credit for the characters he creates to personify each species.
I hadn't noticed it before, but the antagonists from both the vampires and the faeries are female. Bianca and Mavra; Titania, Aurora; Mab, Maeve. On one hand, the overabundance of femmes fatales might be worrying. Then again, for the forces of good we have Karrin Murphy. While she's not as powerful as a vampire and certainly can't take on a faerie queen, she still kicks chlorofiend ass. Harry's lucky to have the help he does.
Summer Knight is the debut of another major theme in the Dresden Files. Harry isolates himself from his friends in an attempt to find a cure for Susan's condition. It's obvious that he can't continue in such a state for much longer; withdrawing from society is seldom a solution (unless you're Salinger). Indeed, Butcher ramps up the conflict in this book to remind us just how much Harry needs friends and allies. In Grave Peril, Harry shoulders a lot of the legwork, and the climax is his alone. The conflict in Summer Knight is on another level altogether: this time, instead of war between the Red Court and the White Council, we're talking a war between seasons, between the Faerie Courts. No matter who wins, humanity loses. Harry can't stop that alone.
Although he seems to dodge a bullet here, this isn't the end of Harry's journey. That's most evident in Harry's conversations with the faerie queens: both Mab and Aurora judge Harry by the scars they perceive on his psyche; the two Mothers were equally creepy in their evaluation. The burden of power—and the accompanying responsibility—will continue to weigh heavily upon Harry.
Are mortals meant ever to confront such power? The fates of both the Summer and the Winter Knights seem to suggest not. Easily overlooked are the changelings, the human-fae hybrids who must choose to become one or the other. Meryl chooses to troll up, preferring to sacrifice her humanity and her life to aid the cause. Lily and Fix take a different path.
I know that Harry gets more powerful as the series goes on. His encounters with various non-mortal agencies leave lasting marks on him, and he receives many mantles or grants of magic that prove a serious temptation. I don't think Harry could ever be a Lloyd Slate no matter how much power he has. Yet his weakness is his protective streak, especially for women. As we saw in Grave Peril, there is nothing Harry will not do to try to save someone for whom he cares, right up to instigating bloody war.
Butcher combines faeries with a murder mystery and Harry's own increasing desperation and destitution. It has some of my favourite parts of the Dresdenverse in it: more mythology on the faeries, a very close look at the power structure of the White Council, and great scenes between Harry and Murphy. Summer Knight does what's very difficult, and manages to keep lots of material balanced and use it to deliver lots of story. That makes it exemplary, both as a stand alone novel and as a part of the overall arc of the series.