If you're only going to buy one book on CSS, I highly recommend CSS Mastery by Andy Budd. It actually has a great introductory section in there, and no one puts into plain English better the reasons why certain hacks are used in certain browsers. Web Standards Solutions by Dan Cederholm is a solid intermediate level book and if you want a good one on usability, I think Jakob Nielsen's Prioritizing Web Usability is a solid, well-researched read. While Defensive Web Design for the Web by 37signals is alright, I think it's a bit dated and I found it focused more on sites that execute certain principles badly. I really wished there were more books on sites that get interfaces right--or even one on evolution of a design with commentary by the UI engineers...but that would be too useful.

I wrote a cursory article about CSS techniques on Particletree.

An Overview of Current CSS Layout Techniques http://particletree.com/features/an-overview-of-current-css-...

It's from 2005, but I think it's a good start for getting your head around the fundamentals. And honestly, while Firebug is a great tool for debugging CSS issues, it won't help you introduce to what's going on.

In the end, there's no crash course into CSS/UI design. It's an investment and even after you get a handle on executing good CSS, it won't necessarily help you create fantastic UIs. Remember, one doesn't always follow the other.