I'm not entirely sold on it being oh so great to be able to carry oodles of books around with you. It makes sense with music, because people flick between tracks on a whim. It doesn't make as much sense with books, because you don't randomly decide to read 6 pages of a book and then swap for a different one and so on.
OK, so if you're going on holiday and you're going to do nothing but read, then that's great, but outside of those sorts of cases I don't think they key point of convenience is the multiple books in one package thing.