Hey all, just saw this post!
The Musopen Book is very much alive, in fact we're just about finished. I didnt see an email from you NLewis or if I did I don't recall it at the moment. We're about to launch a new website in a week or so which is why the page is down. If anyone is interested in helping edit or help with our follow up texts, please shoot me an email aaron  musopen.org
Right now I'm planning out a process to have a proper review system so we can get it used in music departments. It's been pretty difficult since there isnt a formal peer review process for material like this.