I am looking into buying some kind of tablet for school and I was just wondering, from users of these tablets...which tablet/apps have the most textbooks?

Are all the books available on Kindle, available on the Kindle app?

Which one has the most textbooks? I want to be able to never have to buy a textbook. That would be really nice.
And if iBook doesnt have it...will Kindle or the Nook app have it? =)