Should I use pixels, points, ems, or another unit identifier to set font sizes? How do I form elements using CSS? How do I set an item's position on the page using CSS?
The CSS Anthology sets out a question and answer type of resource. Most people using this book will be browsing the topics for solutions to particular problems. However, if worked through from start to finish, this book would even serve beginners well.
The CSS Anthology is complete with full code and illustrations. I find this aspect is particularly useful for trying out different approaches. The book also contains a lot of good tips about compatibility and when CSS is not the best choice.