I’m working on a free culture book. The preview is at https://ocaml-book.baturin.org and the source is at https://github.com/dmbaturin/ocaml-book
It’s under CC-BY-SA so it belongs to the community—it can be a living document that people can keep up to date even if original authors abandon it. It’s also supposed to be a collaborative project, but almost no one is collaborating so far. 