With VHS tapes, and DVDs, we watched programs and movies on television screens. Take away the physical media and replace it with files on a computer, and we're still watching programs and movies on television screens.

With vinyl records, and cassettes, and compact discs, we listened to music on speaker systems and on headphones. Take away the physical media and replace it with files on a computer, and we're still listening to music on speaker systems and on headphones.

But books? We never consumed books through something analogous to television sets or speakers. We held the actual book in our hands, and read the printed pages with our eyes. Replacing physical books with digital books is, in my opinion, a significantly bigger switch than moving from compact discs to MP3s or from DVDs to MP4s. The entire experience has changed, not just how the content is stored.