Ajax
Because I liked the Head Frist book on CSS, I read this one from the same series.
I've been noticing that things are happening on modern web pages that I didn't understand -- menus and panels opening smoothly without page refreshes. Ajax is a combination of technologies such as JavaScript, XML, dynamic HTML and CSS which allow changes on pages without whole-page refreshes. After going through this book, I have a basic understanding of how it works.
New features in Dreamweaver CS3 might make it easy for me to start doing Ajax-type things on my pages. I hope so. The Ajax-like tools in Dreamweaver CS3 are based on the Spry framework and I'll be experimenting with those tools soon.