I just finished reading the eBook version of "jQuery UI 1.8: The User Interface Library for jQuery". I do a fair amount of work with jQuery on pa daily basis so I read this book with a pretty good understanding already of how jQuery in general works as well as how to work with jQuery UI. With that said, I learned a lot more from this book than I was expecting. This book has helped me become a better developer with much more knowledge now of jQuery UI than I had before.
The books starts off with a good introduction and how to get up and running with jQuery and jQuery UI for those that are new. The structure of the library is explained as well as what the theme roller is, etc. The basics of how to use the library are covered like events and callbacks which set you up the for the rest of the book's contents.
The second chapter delves into the guts of how jQuery UI works and honestly it is very detailed and will get you up to speed pretty fast. The core css functionality is explained as well as common questions like: what is a widget, creating widgets, as well as how to style them, etc. You learn invaluable things like how to quickly switch themes or overriding a particular theme.
The remainder of the book deals with the actual jQuery UI widgets and how to use them, each one is complete with example code and good screenshots to get a visual idea of what to expect from the widget. The book ends off with UI effects which of course can give your web apps the "kick" they need to get some good visual appeal. For me the chapter on the Datepicker widget was a great help and gave some good information on how to properly hook it up and what additional features could be used.
I don't often give a five star rating on Amazon for any book, this book however deserves it in my opinion. The book is as rock solid as it gets and the information packed into 14 chapters (about 400 pages) makes for a good learning experience as well as a good reference.