While the book starts out with the very basics of selecting elements and interacting with the DOM, the text rapidly progresses to much more complex uses, including animations, ajax, and how to extend jQuery with plugins and custom functions. It includes non-trivial examples (yeah!) and 'labs' where the reader can get their hands dirty and experiment in a controlled environment.
I keep the book handy as a reference, as I am constantly 'almost' remembering how to do something or other.
My first real project using jQuery was transforming a relatively brittle (and very confusing) homegrown ajax application to use jQuery. One of my colleagues commented that the code now "reads like a story". Bear, Yehuda - you guys are making me look good. Thank you!
"jQuery in Action" grabbed my interest and kept it. This was a great contrast to the previous book I read - "Learning jQuery 1.3" which made me want to pull my hair out at the copious HTML code. By contrast, the longest "jQuery in Action" code example was 3.5 pages and there were only a few approaching this length. The vast majority were significantly shorter and all were easily digestible.
Note that this review is for jQuery in Action 1st edition which covers jQuery 1.3. If you go to the publisher's website, you can buy the second edition which covers jQuery 1.4. While only three chapters of the second edition are available, you can see them as they get written. Plus it includes the e-book for the current edition. If I were buying the book now, I'd choose this arrangement. It's a great idea on the part of the publisher as it solves the "I want to read a good book about jQuery now but don't want the latest edition ASAP."