Isn’t it about the target audience for the book? if you were writing for existing designers who wanted to use CSS then the starting point would be different to a book for beginners to design.

is the book for complete newbies or switchers?

I’ve carved out a niche business just from building pages for designers using CSS and (x)html and most of them tell me they’re okay with using CSS for styling, but not for layout (probably due to all those books that focus on styling) If I were writing a book on CSS for the market that I observer it would be based on how to mark up a PSD using semantic html and CSS and getting it to work cross browser. So my starting point for “about CSS” would be how to use it for layout.

Hmm, not quite what you meant really, but hopefully it’s a helpful comment ;o)