I'm trying to do this with Penflip (https://www.penflip.com). It's like GitHub, but for writing. Just like GitHub, there are public and private projects. Anybody can contribute to a public project by submitting a pull request. A big hurdle is that the concept of 'open source' is a bit foreign outside tech, but I think there is some potential.
Here's an open source Java book on Penflip: https://www.penflip.com/lynnlangit/tkp-lesson-plans