The typical HTML/CSS book is a manual. You are told "To do this, you do this" over and over again, and typically there is an appendix listing all the different tags and what they do.
Sexy Web Design isn't a typical CSS book.
Elliot Jay Stocks takes web design beyond the nuts and bolts and starts talking function and elegance. As important as it is to create code that is compliant, it's even more important to create a website that is easy to use, and is visually appealing.
Stocks focuses on "user-centered design." At every step in the creative process, the emphasis is placed not on how cool the site will look, but on how the user will interact with the site. Stocks reminds web designers that usability is at the heart of web design; a good design will be visually appealing and usable at the same time.
One of the first things you'll notice about Sexy Web Design is the use of wireframe diagrams to test general layouts. Stock recommends using pen and paper to start off with; I use graph paper most of the time for my own wireframes. It's a great way to make yourself think about what the site will look like before you start designing. Stock uses wireframes throughout the book to illustrate the thought process behind various design ideas. Stock also emphasizes the importance of good research before beginning the design. Knowing what the purpose of the site is supposed to be, what users are looking for, etc. will dictate how the design is done. Navigation and interaction are also vital parts of any well-designed website, and Stock goes into some detail in discussing the dos and don'ts of navigation design.
Once the nuts and bolts of function are thought of, aesthetics are considered. Aesthetics have to consider the culture of the company or organization that owns the site, and is one of the aspects of web design where the artistic and creative skills of the designer come into play. The more "cutting edge" a company is perceived to be, the more creative freedom a designer may have. Stock puts aesthetics toward the end of the book so that designers see the emphasis on usability — in designing websites, form must follow function, or the result is a beautiful website that nobody can use. A site that is visually appealing but isn't able to be navigated easily is worthless to the user, and ultimately to the company owning the site.
Most web design books aren't meant to be read cover to cover. They are reference books, meant to be sat on shelves and pulled out when there is a question. Sexy Web Design is a book that should be read cover to cover, as often as possible. The lessons taught in this book are timeless, and invaluable.