MUCH NEEDED! I wrote a book last year and found myself battling with formatting/organization constantly. I was working with a more traditional publisher and the process was mainly microsoft word & dropbox. It worked, but I kept wishing I was using markdown / git. An easy to use interface for reviewers would be great as well (something that doesn't require any knowledge of git).

Press "Start writing" -> "Error". Not the most polished product.

I'm using GitBook for my book, their editor is perfect!

This is perfect!

Looks super promising.

This is fantastic. The idea of branching on new ideas or alternate approaches to a given chapter or section is brilliant, and something I'd been hoping someone would create for years now. The OSX editor is great - although the signup-by-github method was a bit confusing and resulted in me having to reset my password. Really looking forward to using this on my next writing project!

Reminds me of Penflip (originally hunted here: http://www.producthunt.com/posts...). These webapps are always neat, but I think the key to success in the collaborative writing/version control market is to integrate into existing workflows. I don't want to use a new editor when I already love Ulysses, but it'd be great to be able to fork and git-diff projects. (I've only just noticed that Penflip actually has a git-friendly API... interesting).

nice tool for those who are familiar with GitHub.