the best way to learn css is to hand-code it...i would reccomend The Definitive Guide

Some other great resources:

w3c schools
a list apart
css resource guide
CSS Zen Garden (bad***)
jeffrey zeldman
eric meyer
dan cederholm
dave shea
andy budd
brian fusco and akash goel
dave dunkerley
jakob neilson
joe clark
innovative design archive
eyerwire
inform IT
digital web magazine
web standards project

Good luck,
Naveed