Peter is perfectly right with his introduction to the book - "Let me tell you a little about who I think you are: You're a web professional who's been hand-coding HTML and CSS (...)". This sentence, probably, describes most of the home grown HTML developers around the world. If you are working with CSS and you want to know what to expect when it comes to CSS3 this book sound to be quite useful. Peter goes over the features of CSS3 while at the same time presenting them in a structured way. He discuses particular rule, shows examples of the usage, and, at the end of each chapter, summarizes their support within most commonly used web engines: WebKit, Firefox, Opera, and IE. You will find this list again within appendix - this way you can easily check whether particular feature is missing or not within given Web browser.
When it comes to the content, it turned out that I am really a casual user of CSS. There are many rules that I was not aware of. This way, I was able to learn new stuff. On the other hand, I think that material is quite demanding for the reader. As Peter states at the beginning of the book: "The Book of CSS3 helps you leverage the excellent knowledge you have of CSS2.1 in order to make learning CSS3 easier. I won't explain the fundamentals of CSS". This is true indeed. You have to have the knowledge of basics in order to benefit from the book. I suggest getting some other position that will teach you CSS from the scratch before targeting this one. What I can definitely say about the book is it's style. It suits me. Peter simply focuses on the matter itself. However, keep it mind that book is not for a beginners.