Primarily literature so far, and the current Kindle is clearly optimzied for that.
When e-readers are also full-capable tablets there’s no stopping fully colored and even interactive books, using videos as well as extensive hyperlinking.
What if your study books were hyperlinked like Wikipedia and contained videos to explain things that would be hard to explain through text? Wouldn’t studying be more interesting and effective that way? Sure it would.
What if Wikipedia becomes the main source for reference information in schools? Is it already?