There are incredible online annotations for novels by Thomas Pynchon[1] and David Foster Wallace's Infinite Jest[2], which are what I thought of first when they talked about expanding into annotating literature. The monetization potential in that area in particular is huge, looking at Cliffs Notes and the like. It's interesting that Andreessen Horowitz is investing in a company that seems to be solving the community problem rather than the technical problem of mass annotation. It seems like the technical challenges were the main hold-up back in the early 90s, but Wikipedia has shown the power and importance of community building when you're trying to create a universal reference.

[1] http://pynchonwiki.com/

[2] http://infinitejest.wallacewiki.com/david-foster-wallace/ind...