The book is full of code samples that borrow heavily from YUI both in concept and source code, though I wish the attribution was a little more transparent in some places. Still, it shows just how design-pattern centric YUI and other libraries are, which is an important point to make.
This book also suffers from a common problem among design pattern books: sometimes two patterns are so similar that it’s hard to determine when to use one over the other. Is it a facade or an adapter? The line is incredibly thin between the two. I would have liked to have seen a final summation chapter that listed each pattern including indications, contraindications, and related patterns. Ross and Dustin do make some attempts at comparing certain patterns in the book, I just would have liked to see such comparisons called out more strongly.