What I learnt from this book is why you need to go to web standards and use xhtml. It also made me clear why and how to use the <div> tag. This last thing is important as otherwise you might change all your regularly use <font> to divs and spans.
The book is not a guide on css or xhtml alone. If you need a reference you need to read one of the more specialized books on these topics. It is best to describe it as a general overview on, as the title says, Designing with Web Standards, and focuses on the structure and use of xhtml and why you should use it. It also briefly touches typography, doctypes, dom and the other browsers like Linx and JAWS screen reader and how to make your site accesible for these browsers. Zeldman finds it more important to have users with old browser technology being able to read the content then having the page rendered with the perfect layout. This makes a point in my opinion.