I just finished reading DOM Scripting
var and why it matters well.
In retrospect the book was probably a few thousand words "too simple" for me. I'm not an expert at DOM scripting but the XHTML, CSS and stuff was a bit basic. For example, I've already used AJAX, I didn't need to know what it is. Did that sound too cocky? Sorry, I hope you see what I mean.
If you're quite new to web standards, DOM scripting and AJAX and stuff; get this book! Or, you're an old-school web developer still using Flash, tables and popup windows; get this book!