
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.
Nearby shelf: The Design of Everyday Things, Maverick, Founders at work, A Brief History of Time, A Pattern Language, Peopleware, Made to stick, Web Standards Solutions, Designing Interactions, The Pragmatic Programmer, The Mythical Man-Month, Oh, the Places You'll Go!
Other good reads: Blink, Tipping Point, Long Tail, Freakonomics