Forum Moderators: not2easy
Another online resource I highly recommend is the CSS Zen Garden [csszengarden.com]. I'd suggest just browsing through all the different designs to see what is possible with CSS. And when you find a particularly interesting layout, you can look at the CSS file to figure out how it was done.
Looking at code is a good way to learn. To help with that I'd recommend getting FireFox and installing the Web Developer extension. Not only will it let you view a site's CSS, you can actually edit it and see the results live. That's a great help in picking apart code, and seeing the effects of different CSS values on the design.
Offline I'd recommend any book by Eric Meyer.
And don't forget about the CSS forum library [webmasterworld.com]. There you'll find lots of particularly useful threads.