That's fine if you just want to learn University Physics on your own, but in college the professor will usually assign homework from problems in the book, which a comparable book won't have.
Also, for K-12, although the thought of being able to buy books for $15 sounds great I think the start-up/maint costs will be too much. Hardware will increase every year, and hardware requirements for software will increase to match that. You end up with 4-5 year lifespan before you have to buy new iPads because the old ones aren't compatible with new textbooks/educational software. Plus you add in the IT related costs of having to maintain a fleet of iPads along with replacements for breakage/insurance premiums. Most schools use books for many years before they replace them with newer editions.
I'm not trying to down play the how awesome it would be to go digital with textbooks, I just think with the resiliency of books and the cost of hardware this is not going to be the revolution apple thinks its going to be, right now anyway. (But then again, maybe I'm wrong)