I wonder if students would use this to read notes/textbooks cooperatively?

I can't count the number of times I've wasted half an hour trying to figure out WTF an author or professor is talking about, only to learn two pages later that I was hung up on an over-generalized equation, badly-worded simplification, or similar. One yellow sticky note in the margin that said "gets more specific 2pgs later" --which a lot of readers would pencil in for themselves anyway-- could save man-years of time over the life of a text.

It would be useful for authors too. Teaching documents usually have a feedback cycle measured in years (if the loop is ever closed at all), whereas this would allow them to access their readers collective impressions almost immediately.

You could start by adding publicly-available PDF textbooks like:

http://www-stat.stanford.edu/~tibs/ElemStatLearn/

http://diestel-graph-theory.com/

http://www.plasma.uu.se/CED/Book/

(I'm sure there other open texts out there that are less wonky, of course. This is just what's on my menu).