Here's a couple examples...
Here's an interactive Periodic Table. I have a feeling this gives you a bit more than a copy/pasted text file.
And although it's a kid's book, this shows a lot of potential for things they can do with text books.
http://www.youtube.com/watch?v=OWJm7xNtRuI
Imagine a music book that had notes you could touch so you'd be able to hear the tones.. or a keyboard that you can play the written music.. imagine an architectural book that would allow you to move around elements of a building to see if the structure could stay sturdy or a fashion design book that would allow you to experiment with different colours and textures and textiles all with the same article of clothing.
At the moment, it's just iPad that's doing these things. I'm sure other companies will follow suit.
Right now, I just really enjoy being able to have all 5 books I'm reading with me at all times, even when I'm at the local starbucks for a coffee.