Absolutely indispensable. This travels with me almost everywhere, literally. I may not always have a quarter to call home, but I always have my CSS Pocket Reference.
This is an amazing reference and every web designer should have a copy. Its arranged very well and is easy to tote around. Could have used some kind of visuals for the CSS properties, but it is only pocket size. If you don't have a copy, you should really get one!
Only gave three stars because its just a reference (although it is a must own). Take a star off if you're a visual learner and add one if you like dry programming books.
This book has definitely been invaluable to me in expanding my knowledge of CSS over the years. There seems to always be something new to learn about CSS. In fact I was reading through it pretty well every day on the bus one summer.
Useful as a quick reference guide, I find this handy when searching for known selectors/properties. With the advent of CSS3, this pocket reference (covering CSS2.1) is now a bit dated.
This is the second most used book in my library, outshined only by a notebook where I keep notes on all things I'm forever having to look up.
CSS: The Definitive Guide is an awesome reference, but a bit bulky to take on trips, or to have on your desk all the time. This little gem steps in and rescues me regularly.
I also have the first edition, so I can look up the CSS that older browsers recognise.