I think when thet say app books, it's not individual book apps but one app, such as Inkling, with many books. I do see textbooks fading from their original format into app format. It makes sense. Right now with one textbook I have multiple apps and websites that either came with my textbook or I had to buy in addition. If that one textbook was on Inkling it would be a smoother and more interactive learning experience.
The other day I was showing my SO the Khan Academy website and my Inkling app. He said he thought that's how learning is supposed to and he wished he had that when he was in school; it wouldn't have been so boring. There is no need for something so interesting as learning, to ever be boring, especially with the tools available today.
If you haven't checked out
Khan Academy, here is great video showing what it can do.
Inkling App