Some people felt that the Dresden Files books had gotten into a rut, but I never felt that. Sure, there was a formula, but it was tapping into a field of possible storylines that was very broad and that I felt could be sustained a long time. In fact, I was feeling like it was time for Butcher to clean house a bit and tie up some of the loose threads. The ending (not necessarily forever, of course) of the Lasciel storyline just made clear how many storylines had left dangling plot elements that never got resolved.
Nevertheless, I loved Changes, the aptly-named thirteenth book in the series, despite the fact that it deviates from the formula. It's breathtaking to consider how many things that happened earlier in the series became relevant during this book; in fact, there are very few characters or plotlines, even back to the first book, that didn't show up in some form or other, often in surprising and shocking ways. Even more breathtaking is how much changed, and how many "holy cats, I can't believe that just happened!" moments there are. Sometimes you're barely getting past one when another one comes at you from around a corner.
One of Butcher's favorite story elements is piling on so much on the lead character that it gets crazy, while giving him almost no good moments (and those that do happen are almost immediately taken away again minutes later). Butcher's favorite movies must be the Die Hard series, since no one else's heroes take more of a constant battering. Changes doesn't discard this, but it makes it fuzzier. There are a lot more moments of something good happening -- but they're nowhere near enough to make up for the many, many, many moments of something even more terrible happening. More maddeningly, even the good things make you wonder how good they're going to turn out to be in hindsight -- and some of the bad things make you wonder how bad they're going to seem, too. It's a lot more ambiguous and complex than Harry's usual crap-storm.
There are a lot of little things that got brought up in this book in a way that makes them all sound like they were part of some larger, overarching, pre-planned story arc. I have no doubt that many of them were not planned that way originally, but Butcher does an exceptional job making them work out that way.
The tension level starts in the first sentence at about the already-high level he usually hits by mid-book and cranks up from there. It's established early on that anything you took for granted that would never change or never go away or never happen could very well end up happening. There are few if any sacred cows left in the book. This turns out to be key to the tension level building: as you start to see things that could never change changing, could never be lost being lost, the stakes increase and every interaction becomes loaded with more dramatic tension.
There are still plenty of laughs, probably about the same number as usual, but when they come, they're more intense just because of the contrast and the relief they offer.
I'd been warned beforehand that it ends on a cliffhanger in a way that no previous Dresden book has done, and that's certainly true, but I was worried that that meant the main story of the book would not end up being resolved, and that I'd end up wishing I'd waited for the next book to read them both together. I needn't have worried. The cliffhanger is more of a teaser of how the next story will start than an unresolved ending on the current story. I don't think this will spoil you any, because knowing that the story has its ending in this book won't give you any hints of what that ending will turn out to be -- far more than in any other of his books.
It's very intriguing to think what's going to happen next. (Not that we needed that cliffhanger for that to be true -- there are plenty of other things in this book that made "what the heck is going to happen next" come to the forefront.)