This would definitely be a very different way of collaboratively producing books than, say, the wiki model, where you have a central branch that everyone simultaneously collaborates on. Under the git model, everyone would essentially be writing their own book, and if you liked some of their articles you could insert those chapters into your 'own' book as well.

This just seems more natural to me. The wiki model implies that one version is best for all, a git model implies that there are as many good versions as there are authors and the user can choose..

If you start this project, put up its github address here so that the interested (including maybe myself) can fork it.

I don't really have a topic to write about, as I am by far not knowledgable in any subject to write a meaningful book length text, not to speak of my writing skills in general. But feel free to propose some!