Articles about CSS selectors:

• Selectutorial—CSS selectors ~ CSS 1 & 2. "Find out more about selectors including the structure of rules, the document tree, types of selectors and their uses" (you do not need to look through "Selectors in action—a step by step tutorial", but it wouldn't hurt)
• Complete CSS Guide: Selectors ~ A tutorial on selectors, CSS 1 & 2.
• Selectoracle ~ Find out what some complicated selectors (CSS 2 & 3) mean.