It's in the [link|http://www.w3.org/TR/REC-CSS2|CSS2 spec] at w3. Read the whole [link|http://www.w3.org/TR/REC-CSS2/selector.html|chapter 5]--can't wait for sibling selectors to come into widespread acceptance in browsers. WOW! That would make my day.
The other indispensable tool for CSS is Eric Meyer. His [link|http://www.meyerweb.com/eric/css/|personal website] has a lot of good stuff on css--demos in the "edge" subsection. You might also want to order his [link|http://www.ericmeyeroncss.com/|recent book], which opened my eyes to the usefulness of child selectors, among other things.
HTH!