Wikibooks is an effort to do to books what Wikipedia did to the encyclopedia: build them using shared, open source collaboration. I think this could have some real application to texts and class notes.
From the few I checked out, it seems like it's still more idea than reality. Books are bigger than encyclopedia entries, so they will be more likely to be incomplete. This will negatively affect user's perception of quality. Wikibooks could probably use some way of segmenting works in progress from those in production and some community based way to promote books from the draft bookshelf to the production bookself.
Since I found out about this in print, I have to assume it's old news. Still, it was new to me, so it might be to you as well. One question I still have is how this is or could be related to MIT's OpenCourseWare initiative.