While I appreciate the value of just reading books, there are some really cool things that can be done with apps. They could offer a reader even deeper immersion in the story if done correctly.
The book app I read about today is a new version of Mary Shelley's Frankenstein. You can actually change parts of the story by making choices for Victor. They added some meat to the book, of course. The original is only 70,000 words, which isn't really all that long for a book.
Some people would care that they messed with a classic. I don't mind all that much (obviously, I monkeyed around with Stoker's work) because you aren't removing the original from existence. It's still there in all its glory in print and as an ebook.
I know this is probably, technically a mashup, which I have likely expressed distaste for in the past. However, this isn't just slapping new words on top of old words. It's an entirely new art form that isn't going to stop at being pretty and semi-functional. They're going to offer a new level of immersion in the story that books usually can't offer.
So what could a book app potentially offer aside from user influenced story lines? Artwork, of course, maybe a soundtrack, videos, author readings, footnotes, maybe a link into a message board where people discuss the book, share buttons for social networks like facebook, pretty much anything that can be displayed on a screen or played as audio.
The only potential downfall of the book as app is the expense of creating one. In order to justify the expense, whoever is sponsoring the endeavor would probably want to be sure they're going to recover any expenses. Not all books turn a profit.