Genuine question: isn't that targeted at writing (and then publishing) actual books, rather than relatively (compared to a book) short documentation?
It is but it is also used for documentation. The Redux documentation use gitbook for example http://redux.js.org/
Or GitHub's wiki
I personally don't like the Github wiki because you cannot transactionally commit to the repo and the wiki. I like adding documentation and code in one PR so it can be reverted together if need be.
GitHub wiki's are just Gollum in front of a git repo. Run your own Gollum instance pointed at the project repo.
I guess. It seems easier to just have Gitbooks host it for free though.