Why help complete this? If they would be willing to have an open source D2 out there, they can just release the original source code. It would seem even more "ideal" to me.
It is a work in progress. The current version is a rough draft with many problems and placeholders. I am using gitbook and github to keep track of versions of this website and the book itself.
You definitely do not get that impression just looking at the homepage. Beware!