Changes is, in its own way, an ending point for the Dresden Files. More precisely, it is the end of the "old" Dresden Files and the beginning of the new. The major plot arc for the first ten or so books of the series, the war between the White Council of Wizards and the Red Court of Vampires, comes to its dramatic close. We also bid farewell to the majority of the original series' trappings like the Detective story elements, its Noir trappings, and the idea this is anything but a very high fantasy urban fantasy series.
Some of these changes are for the better.
The premise of the book is Susan Rodriguez, the first love interest of Harry Dresden, has returned from a lengthy absence to inform her ex-boyfriend he's a father. Rather than bring them together, the discovery that she cut him out of his child's life without even allowing him the decency of knowing destroys any respect Harry has for his now-vampiric former lover. Worse, his child has been kidnapped by the Red Court of Vampires for an evil ritual which will probably result in her sacrifice.
The Red Court is pulling out all the stops to destroy Harry, having grown absolutely gut-sick of the constant never-ending interference of our protagonist. This means attacks on Harry from supernatural, mundane, and spiritual corners. For once, they are portrayed as being every bit as deadly as their reputation implies. Watching Harry's life begin to disintegrate underneath the onslaught of terrible things is some of the most tense storytelling in the series.
Changes makes excellent use of continuity without feeling the need to bog down the reader in irrelevant details. As Harry's life slowly unwinds, he proceeds to call in just about every single favor and ally he's made in the past ten books. He acts intelligently, with conviction, and proactively rather than reactively--at least to dealing with the problem at hand. It's one of Harry's finest moments, taking charge of his destiny to get back his daughter.
Much of this novel is devoted to the moral question of what to do in order to accomplish your goals. Nietzsche formed an entire moral philosophy around this concept and what you should be willing to do. Here, Harry makes a statement he will do anything to save his daughter's life. Anything. That includes making a deal with the Queen of the Winter Fae and, if she refuses to help, darker sources of power. There's some genuinely shocking moments and they are all the more effective because you wouldn't think Harry would stoop to those levels.
The ending of Changes is, sadly, something readers will never be able to experience the same way fans did when the book first came out. It was so shocking and unexpected everyone was seriously considering it to be the end of the series. Now that new volumes of the series are out starring Harry Dresden, some of the initial impact has been lost. Still, it's a surprising ending and more effective for that.
In conclusion, this is one of the best books in the series but it's also one which is terrible for getting on the series with. Much of its appeal stems from the great use of continuity and previously established characters. Despite this, it's so well-written and such a classic story you could jump on it if you wanted to. The humor, action, and villains are great in this book. There's also a lot of powerful terrible moments like the resolution of the Harry/Susan plot.