Pro JavaScript Techniques
Pro JavaScript Techniques is the ultimate JavaScript book for the modern web developer. It provides everything you need to know about modern JavaScript, and shows what JavaScript can do for your web sites. This book doesn't waste any time looking at things you already know, like basic syntax and structures.
This is a very easy way to check if an object has a given property. Easier than checking if (obj.foo != null) or some such, which is what I've been doing :( From Flanagan, 5th ed, p. 108.; but I actually ran across it in the source of this page: http://www.gtalbot.org/DHTMLSection/DynamicOpacity.html, thanks to Gerard Talbot on clj.