CSS: The Definitive Guide
An absolutely magisterial work that explores every last corner of the CSS spec. The advancements that have come along since I learned CSS are amazing – you can almost write executable code in CSS now. This book explores every edge case, no matter absurd, to probe every last corner of the spec.
I admit to skimming some chapters, because some of it is so complicated (ahem…transforms), that you would really need to have an active project you were working on, and could apply it to.