As the title suggests, this book is aimed at higher traffic websites.
Is the book assume that I already have a high traffic site, or does the book is aimed at helping me develop such site that draws traffic in terms of design seo?
The book doesn't cover ideas on how to drive traffic to your website. There are considerations that affect not only SEO but accessiblity and best practises that are talked about throughout the entire book, as this is important if you are dealing with large websites with business needs and that need to cater for a large audience. It assumes that you either have a website that is aimed at high traffic, plan on having one, or would like to prepare your website for that possibility in terms of CSS.