What I like about it is the way they track progress through the book. They are doing so on the client side which is probably the part that interest me most about it. I assume it won’t work accross devices or browsers, but there may be ways of achieving that in a way that people can integrate it on the client side. Kind of like disqus how disqus normally works, but for keeping track of where you are.
Other than that, I think it is a good tool to use for creating a book like experience with some level of interactivity.
What is missing for me is the social bit, the sense that other people are also working through the same book. I can’t see that in their JavaScript example.
Adding togetherJS to this may be interesting. Adding comments or links to a forum would also be useful.