Professional JavaScript for Web Developers, 2nd ed. by Nicholas Zakas is a great book for getting further in depth with the language. A great reference which covers a lot of cross-browser differences (and solutions). There's tons of information in this book, and it has helped me a great deal at work. I mostly use this for understanding others' JavaScript and also as a reference. Touches on advanced topics, but doesn't go too in depth on advanced design and usage, but covers plenty good information which experienced programmers can apply. The cover price is $50 - I was lucky to have this covered as a work expense. If I had to pay out of pocket I would say it's worth $30-$40. As someone who had touched on JS several years back, and was mostly afraid of it, I now feel a lot more comfortable developing and maintaining it. I rate it 4.5/5 stars (lack of more advanced design and usage), but well worth it for Beginner to (slightly) Advanced.
Covers multiple methods (with pros and cons) of OO designs, different types of Event mechanisms, the DOM, Xml, Ajax, etc. Even includes information on efficiency and speed.
[edit] I've read reviews of the 1st edition of this book, and many seem to be disappointed with it. Please remember that this review is for the 2nd edition.
