With ebooks on tablets, we could easily have video or interactive elements as the book cover. There's no need to stick to the static image format we find on printed books. Of course, there's no need to stick to the static text format for the main body of the book, either, which is where the traditional notion of a "book" starts to break down. The medium changes the message, or it can.
Mokalus of Borg
PS - There is an emerging group of "book-like" apps, especially on the iPad.
PPS - Obviously they're harder to produce than textual books.