[cben]

I don't think this editor requires subscription? You can skip login ("not now" or something like that at the bottom), and work with local git repos; I suspect you can also use coax it to work with private remote repos (Book -> Repository Settings... ?) without logging in, but haven't tried.

You won't get GitBook's online services but their build toolchain is fully open, can be run locally.

Aside from being proprietary, this seems a neat local markdown editor with built-in git support. I too would prefer something FOSS and more flexible, esp. for a huge project like a book, but I can see myself recommending this to people.