For me, this "Book" (which is actually just a series of web pages) failed the usability test when I got stuck wondering "where is the table of contents?" (usually in a side bar). After clicking "read the book" expecting a book i.e. a pdf, or something I could buy, I eventually discovered that the ToC is only present on the front page, prior to clicking through to read. Furthermore, the ToC is completely unlabeled, and inside a box which makes it look like a footer. A simple H1 "Contents" would have been helpful.