Last night, I finished reading jQuery UI 1.6 by Dan Wellman (published by PACKT Publishing). Released in January 2009, it's slightly behind the times - jQuery UI 1.7 just came out a few weeks ago. But, even if it's not completely up to date, it is still a great book that covers the fundamentals of the jQuery UI library, all the existing widgets, and even touches on the CSS widget framework (something that was really fleshed out in v1.7).
jQuery UI 1.6 is the perfect mix of reference manual and real-world tutorial. It covers every aspect of the jQuery UI behaviors, utility classes, and widgets in a systematic way; but, it keeps it interesting by iteratively applying new ideas to easy-to-follow examples. It does a great job of getting you familiar with all the configuration options, methods, and events, and really gets you to start thinking about where you might apply such aspects in your own programming.
You'll have to look at the book or the documentation to see what each of these events represents; but, with such a robust event model, the jQuery UI library gives you ample ability to monitor and react to all aspects of the UI library behaviors. The jQuery UI library black-boxes its implementation, but it gives you plenty of opportunity to follow along and even modify the way things are working.
The jQuery UI library is narrow in its API but extremely deep in its functionality. Learning how to leverage its entire set of features is not going to take place overnight. But, for anyone looking to get more comfortable with the jQuery UI library, I highly recommend jQuery UI 1.6 by Dan Wellman as a starting point. Like I said above, it's the prefect mix of real world tutorial and reference manual. It will get you solid on the basics and itching for mastery.