Grab a free chapter of the latest YUI book from Packt Publishing, "YUI 2.8: Learning the Library." The free chapter covers something we all use a lot, menus. I'm checking it out now, and suggest if you use, or are thinking about using YUI, you do the same since it's free.
I've been using YUI for about 4 years now, and love it. I love the solid foundation it provides for me to build my own interfaces on top of. I love the polished UI elements it provides as well. I'm looking forward to reading this book in its entirety. Check back soon for a full review of it.