The very moment we saw the first hyper links in documentation, around 1985, we saw that the textbooks of the future were not just about text, but about many different kinds of media.
GigaOm writes in an excellent article:
"... As we predicted, Apple on Thursday introduced a new toolset for publishers and authors to create and distribute digital textbooks. That wasn’t too much of a surprise. But what’s become more clear after hearing Apple’s pitch, which is aimed at K-12 school teachers, school districts, homeschooling parents, publishers and students themselves, is that the future of textbooks is not a book at all.... "
But what about their unprecedented and greedy license agreement?
Read the article, very well worth it as an introduction to the future of 'text'books.