I have been using CSS as a way of formatting web pages and have found the following resources to be invaluable in my studies:

Books:
Eric Meyer on CSS
More Eric Meyer on CSS
CSS 2.0 Programmers Reference by Eric Meyer
The zen of CSS design by Shea and Holzschlag

Eric Meyer's books are very hands-on step-by-step training and have accompanying websites where you can download the files and follow the steps. Highly recommended.

Websites:
Zen Garden website - a great place to learn to appreciate CSS design
http://csszengarden.com/?cssfile=/151/151.css&page=0

CSS page validation site - a great tool
http://jigsaw.w3.org/css-validator/

Westciv is a great resource for CSS coders
http://support.westciv.com/

Westciv produces a fine piece of software I use as a starting point for my CSS sites. It is called StyleMaster; it has excellant tutorials and documentation.
http://www.westciv.com/index.html

There is much more out there....just do a Google search for CSS...enjoy!