Interactive e-books might be used for all kinds of entertainment, as well as learning. Imagine an e-book that shows anatomy to medical students, with various suturing techniques, for instance.
Actually, I'm guessing that textbooks and other instructional books will be the first to go interactive in significant numbers. They have the best chance at return on investment right off. Kids will grow up with 'em. They'll interact with books differently than we do; they already do with any technology, as we fogeys can see. Not that some of us fogeys can't adapt, lol.
Personally, I'd like interactive e-books in foreign languages, biographies, history books, various other nonfiction. For instance, if I'm reading a biography about Martin Luther King Jr., I could could read about him, see his speeches, hear what others said about him in historic context, see snippets of protests and arrests, and so on. Clickable parts of the book could take me to other figures of his time, if I were interested in reading further, and so on.
There are lots of possibilities in books about art, architecture, astronomy, investments and so on, which are also my among my interests. If I read about investments, I could click on comparisons and further delve into specific sectors, methods of investment; I could build a profile of my investment style and filter parts of a book to my preferences. If I read about Frank Sinatra, it would be great to hear and see him and the rest of the Rat Pack, and so on. I could look at a travel book with multimedia that included history, photos and video, maps and so on, and interactively filter to my tastes when visiting a city. And I could take all such books on the go with my hundreds of other e-books.