Someone with legal expertise should go over this looking for something that's less obviously harmful. I can't imagine Apple's legal team not realizing this would come out.
The only reason it would be unenforceable is if the terms are unreasonable. Unreasonable is pretty specifically defined in the common law, and it doesn't just mean that you and I don't like it.
In this particular context you would probably have to prove that there is economic duress, which might be a valid argument if Apple owned a monopoly on all textbook producing software and electronic textbook sales - which as of right now, they don't.
So barring something else more monstrous lurking in the EULA I'm not convinced that anything can be done about it (in court).