I do thing there are applications to a more multi-media centric book. Think of a textbook, history book or travel book that was augmented with in line video or audio clips. This is still a book in every sense of the word, but has some additional content inserted instead of just tables or pictures.
Don't get me wrong, I'm primarily a reader and I personally hate audio books, but the ability for a device to augment the reading experience in this fashion could be pretty cool. Maybe by the time I retire something like that will exist.