These comments (and the star rating) are very specifically from the point of view of someone who wants to add some interactivity and AJAX to web pages using the most straightforward efficient method, which is with JQuery.
THE GOOD: These guys absolutely know JQuery and JavaScript. They are fluent experts and authorities. They know the minute details and the inner guts. Also, they put a great deal of effort into this book. They built some good downloadable learning tools for the early sections and thought about the organization of the material.
THE BAD: Too few examples. Often complex commands are introduced without even an example to illustrate the syntax.
(FOR EXAMPLE, early on when selectors are discussed, they introduce a selector that requires quotes. That selector itself must be contained in quotes. They never show how the quotes within quotes syntax is handled).
The examples that are included are often not simple or straightforward. To illustrate AJAX their example gratuitously includes a custom plugin. I'd much rather have more examples of variations of the AJAX calls in the AJAX section instead of one long clunky example that illustrates only limited cases of the various jQuery Ajax methods. In several cases, The most complex JQuery method with more than a dozen possible parameters is simply listed with the parameters barely explained with no examples at all. Maybe if I was a professional JavaScript programmer a lot of the left out stuff would be trivial or obvious. But it wasn't for me.
In other places there is a surfeit of unnecessary technical material. The chapter on events, for example, starts off with long sections on the DOM event model and cross browser issues without a HINT that those issues aren't material to the JQuery user (that's the point! JQuery handles that stuff so I don't need to know).
Stylistically, these guys seem to be inspired either by ad copy (there is a ridiculous surfeit of exclamations!) or by programming blogs. They have the a fondness for jargon and dogma that seems to be the morass of the self-educated technophile. Many pages are wasted with examples of How HORRIBLE it was in the days before jQuery. In some sections (like the beginning of the AJAX section) they elaborate on the complexities of browser differences for AJAX calls. One of the most complicated sections in the book, only to show that you really don't need to know any of that stuff thanks to The Miracle of jQuery! (!))
INTERNET EXPLORER: As far as these guys are concerned, Internet Explorer is a bastard stepchild marginal fringe case. They seem embarrassed and appalled that they have to mention it within their pristine pages. OK. They don't like it. But more than half the browsers out there are IE and IE 8 continues to have its own quirks and not follow standards. DEAL WITH IT. JQuery itself has very much code dedicated to sorting out IE issues. It would be nice if the authors would hit that issue head on. A simple list of the various things you can do with JQuery that fix previous browser difficulties that required different code (CSS properties or JavaScript DOM issues) would be nice. Dealing with Internet Explorer hassles (and cross browser hassles in general) is one of the great gifts of JQuery.
Marginalizing that gift because of a distaste for the major browser (like it or not) just is not helpful. I'm not saying they deny the existence of IE. They just don't make it a focus at any point.
FIREBUG: The authors wait till quite late in the book and then treat it as a sort of aside. Firebug is THE javascrip debugger for Firefox. The authors (in their brief aside) acknowledge that no one should be writing anything in JavaScript (and so, in jQuery) without using a debugger. Firebug is a fantastic learning tool for jQuery and makes the downloadable "lab" pages the authors provide more or less unnecessary. Since the authors clearly use Firebug themselves and acknowledge how useful and important it is, why do they barely give it a mention? I bet they used it when they were learning jQuery.
I don't like giving a book like this which clearly shows the earmarks of expertise and hard work a negative review. It may be the best JQuery book out there (I haven't read any others yet), but this book seriously needs some editing. Either that or I am simply the wrong audience. I do believe a professional JavaScript programmer would get more out of this than I did.
But in any event, the book should decide if it wants to be a reference, a tutorial, or both. I just think it's not a great introduction to a great subject. And I know it's a lousy reference, because I tried to go back to some chapters to look up syntax. Hard to find. Hard to read. And few examples.

A good web development framework anticipates what you need to do and makes those tasks easier and more efficient; jQuery practically reads your mind. Developers of every stripe-hobbyists and professionals alike-fall in love with jQuery the minute they've reduced 20 lines of clunky JavaScript into three lines of elegant, readable code. This new, concise JavaScript library radically simplifies how you traverse HTML documents, handle events, perform animations, and add Ajax interactions to your web pages.
jQuery in Action, like jQuery itself, is a concise tool designed to make you a more efficient and effective web developer. In a short 300 pages, this book introduces you to the jQuery programming model and guides you through the major features and techniques you'll need to be productive immediately. The book anchors each new concept in the tasks you'll tackle in day-to-day web development and offers unique lab pages where you immediately put your jQuery knowledge to work.
There are dozens of JavaScript libraries available now, with major companies like Google, Yahoo and AOL open-sourcing their in-house tools. This book shows you how jQuery stacks up against other libraries and helps you navigate interaction with other tools and frameworks.
jQuery in Action offers a rich investigation of the up-and-coming jQuery library for client-side JavaScript. This book covers all major features and capabilities in a manner focused on getting the reader up and running with jQuery from the very first sections. Web Developers reading this book will gain a deep understanding of how to use jQuery to simplify their pages and lives, as well as learn the philosophy behind writing jQuery-enhanced pages.