I like to joke that my approach to editing CSS goes something like this:

1. Right-click, 'Inspect Element'
2. Find the appropriate selector
3. Mash the keyboard until it works

Needless to say, I can always use a little help when it comes to web design. This book, written by Jon Duckett, is a great resource for such knowledge. There's nothing contained within it that you can't find on the web or in a million other books, but it covers the basics very well, everything is presented beautifully, and it looks great on any bookshelf.

If you want to get a feel for the book before buying, Jon has put up a sample chapter for you to peruse if you like.