I've had a chance to read through the latest book on YUI, titled YUI 2.8 Learning the Library. The book was written by Daniel Barreiro and Dan Wellman, and was published by Packt Publishing. For the impatient that don't want to read the whole review, and want to get right to the goods, here they are. The book has great coverage of the YUI library, including all of the popular widget like Calendar, Container, Autocomplete, DataSource, DataTable and more. If you'ren new to YUI and looking for an overage and how-to for the library, this book would serve you well. If you're an experienced YUI developer, you probably won't get a whole lot out of this book that you don't already know, or couldn't gain through reading YUI's online examples and API.
If you read through this book in it's entirety, you'll come to understand the main reason I love YUI, that it's not just a collection of widgets and utilities. This book explains the full feature set of the YUI library, and you'll realize that it's the perfect foundation to build on top of. The author's do a really great job of showing in depth examples that teach you how the components work. I particularly liked reading the chapter on DataSource and DataTable.
If the online examples provided by Yahoo! leave you wondering how things are working, this book fills in those gaps. Most of the book is targeted to beginners and intermediate developers.