Recently I read Yahoo User Interface 2.X Cookbook by Matt Snider.
Here is what I think about it:
In contrast with other books that have lengthy sections of text explaining all the details of some topic, followed by a huge blocks of code towards the end of the chapter, this book is not written that way. It is organized into sections covering many different areas of YUI2 framework. Within a section you'll find headings for the topics "Getting ready", "How to do it", "How it works" and "There's more", a series of instructions that spell out exactly what to do for a sample scenario.
"Getting ready" shows what you need to have already setup in your environment.
"How to do it" is a section showing coding steps you need to take to achieve your goal.
"How it works" is a section explaining step by step what the code given in previous section does.
"There's more" is a section giving additional notes and examples you may find useful in further development.
I believe that YUI2 Cookbook is good for an experienced web developer who is tied to YUI2. However, I can't wait to see the YUI3 Cookbook. For sure YUI2 is a pretty good library, but YUI3 has way more potential. I wouldn’t suggest to anyone to start any project using YUI2 at this point, given that YUI3 is out there in a pretty good shape and YUI4 is on its way.