Not that the faeries aren't some pretty mean SOBs; because they are. A combination of Edmund Spenser, William Shakespeare, and traditional Gaelic folklore about the sidhe give us elves and faeries that are twisted, alien, mercurial and cruel at the best of times; downright destructive, spiteful and murderous the rest of the time. Dungeons & Dragons could take a page outta this playbook, and maybe a few less out of Tolkiens, and be vastly improved in my opinion. If I've gotta deal with elves, this is how I want them. Against the sidhe conflict, in to which Dresden is inexorably dragged, we also have the backdrop of his continued tension with the White Council (of wizards) and the Wardens, as well as the still freshly declared war against the Red Court of vampires.
Butcher likes his sidhe and the rest of the fairies, apparently. Not only have they at least made some kind of appearance (even if only a cameo) in all of the books so far, they continue to do so. While this book is the only one (to date) to treat them as the major antagonist, they continue to occasionally play significant roles in the books to come, especially Mab, the Winter Queen (a name coined by Shakespeare, it appears, in Romeo and Juliet instead of A Midsummer Night's Dream like you might expect (although her Summer Queen counterpart, Titania, comes from there) where she's just an off-hand reference).
Otherwise, the book is a little light on recurring characters; Michael Carpenter and Thomas Raith aren't even mentioned, and Karrin Murphy makes little more than a cameo herself. However, some important characters are introduced, like Mab, the Merlin, Ebenezar McCoy and the rest of the Senior Council of wizards. There's a surprise appearance by another character, but I won't spoil that here. It's big news. Plus, Dresden finally tells Murphy the way things really are, and brings her into the loop. The Alphas, specifically Billy Borden, also feature a bit, and are shown to have matured significantly since they appeared in Fool Moon.
This book's fun because it departs just a bit from the tone and feel of the others. Like I said, it's got a bit more of a fantasy feel, but not too much. There's still a mystery to be solved at the heart of the book, and Dresden does his usual routine to solve it. There's a climactic showdown in the lands of Faerie itself, that betray Butcher's ambition (still unfulfilled at the time) to write High Fantasy. Well, he had written it; to sell high fantasy is probably a more accurate a phrase.
In other words, Summer Knight is a very competently done iteration of the Dresden formula, and one that deviates a little bit in tone, but not much in form. It lays the foundations for much of the Dresden activity that follows without being groundbreaking in and of itself.