When I decided to reimplement 96db.com to run under Django, I read Castro's HTML, XHTML & CSS Visual Quickstart Guide, and it fulfilled the "visual quick start" portion of its title—it got me up and running quickly with reasonable results. However, after several frustrated hours spent tweaking style parameters, I still had trouble getting the layout and presentation I wanted for my site.

In Don't Make Me Think, Krug strongly recommends Eric Meyer's writing on CSS, so after finishing that, I sought out CSS: The Definitive Guide on Amazon. I'm glad I did.

Meyer's book is perfect for programmers. It describes the models that underly CSS thoroughly, along with the algorithms that browsers use for layout. Much more so than Castro's book, it leaves me confident that I know what's going on.

While I was reading, I tweaked a few bits of my site's CSS, and I got a lot closer to my original intent. If you really want to understand CSS, this seems to be the book to get.