I found the book itself to be quite well written. It begins by making no assumptions regarding your knowledge of JQuery (although it does assume you grok JScript) and starts with the most basic steps all the time pushing toward more powerful usages of the library.
With numerous real-world client-side issues addressed, you are bound to find a solution to 90% of your questions in the first few chapters. JQuery is also extensible, and many people out there are continually writing libraries on top of it.
This book dedicates the last few chapters to a review of some of the more common\popular libraries and assists you in understanding where you might leverage it in your projects.
All together, if you have never touched JQuery and are interested in learning something about it, this is a great book for you. If you already know a bit about the language, you are probably looking for something else though.