It pains me to admit that despite the hundreds and hundreds of hours I've spent learning and applying CSS, I have never read a book on the subject. Until now.
This and CSS: The Definitive Guide by Eric Meyer seemed to be the best books available. A new edition of The Definitive Guide is being written as I type this. So I bought a used copy of an older edition of that an a new copy of the 3rd Edition of this book. I couldn't have been happier with my decision.
This book assumes no previous knowledge and covers CSS from the ground up. The more CSS you've already written, the more you'll get out of the book, but that's true of any subject.
I really enjoyed finally getting a thorough explanation of the following CSS building block concepts: the box model (with emphasis on the "border-box" type of box sizing), the line box (the anonymous horizontal box created by a line of text and/or inline boxes), margin collapsing, and the most exciting new one: flexbox.
The "flex" display type, AKA "flexbox" has truly changed the entire CSS equation by eliminating some of the most egregious layout inabilities of the languages. CSS Mastery has an excellent overview of flexbox. It's a big, evolving topic, so some online reading is also needed to get a complete grasp of the subject.
There are a great many examples in the book. It manages to be brief and still contain a ton of practical advice on topics such as typography, responsive design, and forms. I've already used it as both a learning guide and as a reference.
It's an excellent learning guide and an okay reference. (Online resources such as MDN (the Mozilla Developer Network) and Stack Overflow tend to be handier references anyway since they are searchable.)
I did find at least one silly mistake in the book: a screen capture clearly showed the wrong example - leaving the correct result up to one's imagination. I'm not sure how multiple sets of eyes managed to miss that! It's so obvious, I don't think it will confuse anyone an undue amount.
Overall, I heartily recommend the Third Edition of this book for anyone who wants to properly learn how to use CSS in the current landscape. You don't need anything more up-to-date than this.