I’m also working on a free culture book. The preview is at https://damien-guichard.developpez.com/downloads/Algorithmic-with-OCaml.pdf
It’s under CC-BY-SA.
Planned chapters includes : Records, Type polymorphism, Modules as functions, Conceptual graphs.
The reason why i don’t contribute to @dmbaturin’s effort is that my main topic is algorithmic, ocaml is more a good way than a goal.