Hi again J
I started working with DHTML 6 years ago(2000). The Javascript book from O'Reilly is probably the reason why I got so interested in it. That book really inspired me back then. The DHTML book is however the book I refer to most these days.
To participate in a forum like the forum here or the new one we are about to launch tomorrow() is also extremely valuable in my opinion. You learn from others, and you learn from teaching others.
A basic understanding of CSS and HTML is also extremely important. If you are familiar with how the HTML tag works and how to style them by use of CSS, then you're on the right path. DHTML is really just a way of manipulating CSS rules. For example: In my drag and drop scripts I have set a HTML tag(<div>) to have position:absolute; Then I am just updating the elements x and y position on the screen according to the mouse cursor(i.e. "faking" a drag process).
The pages at
http://www.w3schools.com/css/default.asp
http://maxdesign.com.au/
http://www.csszengarden.com
could be worth bookmarking.
Good luck with your DHTML. It's really fun to work with