While I definitely began to get more comfortable with jQuery, my knowledge was a bit scattered. There were some important concepts I just wasn't getting. Learning jQuery 1.3, by Jonathan Chaffer and Karl Swedberg, is an excellent book. I feel like I finally have a proper understanding of how jQuery works and also some of the philosophy behind the mechanics.
Probably the biggest area I needed help with was selectors. The book starts off with a very in depth, very thorough description of this feature. I finally think I get them. Am I able to parse every selector I see? No. For me, selectors act like regex. I know basic regex but at times I have to break them down into components to really get it. This is how I approach selectors now. It's made a huge difference in my ability to work with jQuery. I'm no longer just guessing at the selectors.
After selectors, the book goes deep into effects, events, DOM manipulation, and Ajax. Each chapter contains numerous examples and really gives you a great feel for the feature set. It really became apparent just how scattered my knowledge was. I really being surprised, on multiple occasions, when I found that jQuery fully supported something I was sure didn't actually exist.
Following the 'core' chapters, you then get chapters that go deep into particular uses of jQuery. Things like forms handling and table manipulation. Pretty much the exact kinds of things I think most people are looking to use with a JavaScript framework.
Both plugins, and plugin development, is also discussed. While some good plugins are recommended, I'd like to see a bit more talk about selecting plugins. I still think the plugin system can be a bit daunting to new users. Seeing 200 different options for lightboxes is both good and bad. I know for me it was a bit bad as I had absolutely no idea what plugin would be best for the case at hand. This really is more of a complaint in general as opposed to a critique of the book. But perhaps some discussion could be made of how to recognize what is possible a good plugin. Even something as simple as - was the plugin update recently? Is the author responsive to queries? Etc.
Overall, this is a darn good book. Probably one of the best technical books I've ever read. It's a night and day difference for me in terms of how comfortable I feel with jQuery. If you have any interest in jQuery, or even if, like me, you've been using it but would like to get better (a lot better!), then I highly recommend picking it up. (And if you do it via the affiliate link above, I get a little kick back. :) Also note that the publisher has a sample chapter online here: http://www.packtpub.com/files/learning-jquery-1-3-sample-chapter-4-effects.pdf
p.s. One last detail. As we know, we technical books, things can get quickly outdated. Learning jQuery 1.3 was published in February, and as far as I know, is the most up to date book on the framework.