I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
I think this book is a good companion to DOM Scripting by Jeremy Keith (reviewed here). They both emphasize best practices like unobtrusive scripting and progressive enhancement. The intended audience is also the same.
Overall, I am enjoying the book so far and I have learnt some things that I may have conveniently skipped over in the past.