You have not read a truly digital book until you've read The New World. The novella is about a woman trying to reclaim her dead husband's head from a cult-like cryogenics company. But more than that, it is the most ambitious attempt I've seen at exploring a future where books lack physical form and are better for it.
Consider this possibility: A reading experience on the iPad that doesn't awkwardly mimic a newspaper or try to be some hyperkinetic cousin of a paper magazine. One that combines text and multimedia in a distinctly new way. That's the Atavist.