Is it down? I cannot access it.
You can also fork the repo, enable Github Pages, and then access it at yourgithubname.github.io/python-opengl/book.html
I just made a Github issue suggesting to the author, if he's not opposed to using Github pages himself: https://github.com/rougier/python-opengl/issues/38
“Last point, I wrote the book in a kind of modern Kerouac's style such that you can download it once and continue reading it offline. Initial loading may be slow though.”
I do think the entire book is on a single HTML page