If you are new to CSS and looking for a gentle introduction to CSS3 and HTML5 that assumes no prior knowledge, I recommend considering CSS3 The Missing Manual. I purchased the previous version of the book and have since donated it to the local public library. When it came on the list of available books from the O'Reilly Blogger Review Program, I jumped at the chance to get the latest e-book version so I could add it to my list of reference books on my iPad, etc.
Learn CSS From Scratch
CSS3 The Missing Manual is essentially for learning CSS and even HTML from scratch. The book even points you to various editors, tools, and other online resources so you can begin learning web design. This is clearly a book for beginners learning CSS for the first time. The book starts off explaining CSS, style sheets, and the distinct roles of HTML and CSS. You then learn how to use selectors to identify what you want to style and the power of inheritance and cascading styles.
Once you have learned the fundamentals you start applying those fundamentals to style various parts of a web page, whether it be plain text, navigation, photo galleries, lists, tables, forms, etc. Each chapter has one or more tutorials at the end to test your knowledge.
Towards the end of the book you start using the CSS to create page layouts. This is where you start learning float-based layouts, responsive web design, media queries, flexible grids, and other more advanced CSS techniques.
Not a CSS3 and HTML5 Specific Recipe Book
I have numerous books on CSS and CSS3/HTML5 and they broadly come in 2 different flavors. You have books, like CSS3 The Missing Manual, which are more about learning CSS and have been updated to include some aspects of CSS3 and HTML5. These books are aimed at beginners who want to learn CSS from scratch and appreciate the repetitive reinforcement of the material and longer descriptions.
You then have those books that are mainly recipe and CSS3/HTML5 specific books that aim to get educated web designers up-to-speed on the latest features of CSS3 and HTML5. These book are more terse and example-driven. This is not CSS3 The Missing Manual.
Therefore, if you are completely new to CSS and want a longer narrative on how to use CSS to style web pages and create page layouts, CSS3 The Missing Manual is a goog choice. If you are familiar with CSS, more interested in how to apply the latest standards, and looking for more of an example-driven recipe book, this book is not a good choice.
Lots of Good CSS3 and HTML5 Books
As I mentioned earlier, there are lots of good books on CSS3 and HTML5 and there is quite a bit of overlap. If you are a new web designer, you'll probably have a tough time finding 1 book to rule them all. Each will come with their own assumptions, specific topics, and limitations. For the beginner looking for an introduction to CSS, you certainly can't go wrong with CSS3 The Missing Manual.
If you are looking for a book more focused on HTML5 from the same series, I enjoyed reading HTML5 The Missing Manual. It has a chapter on Boosting Styles with CSS3 more geared to those who know CSS and just want the new parts of CSS3. In addition, the book has a lot of great information on programming with HTML5.
Again, I read a lot of books on CSS3 and HTML5, and there are lots of good ones out there that have a lot of overlap and distinct topics. You'll need to gather a few to really appreciate the subjects as a whole.
At the risk of being way too redundant, CSS3 The Missing Manual is a wonderful book on learning the fundamentals of CSS and even HTML. It is a beginner book and you will no doubt need to pick up additional books that focus specificly on CSS3 as well as those aspects of HTML to complete your knowledge. One book won't do it for real-world knowledge, but CSS3 The Missing Manual will get you started in the right direction.
Learn more about CSS3 The Missing Manual on the O'Reilly Website.