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

Looks super promising.

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).

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).

This is perfect!

nice tool for those who are familiar with GitHub.

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

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!