When it comes to progress on the e-learning curve, one could easily argue that the music industry is well ahead of the book publishing industry. Napster and other illegal file-sharing tools forced the music industry to wake up and smell the coffee. Apple then stepped in with the iPod/iTunes combination and promptly ruled the world...for awhile...
As this BusinessWeek article notes, the pay-per-song model might be living on borrowed time. All you can eat subscription models like Rhapsody are gaining popularity. It makes sense to me. After all, I don't care how much music I "own"; I just care how much I have access to. Would I pay $12.99/month for the right to fill up a 30-, 40- or 80-gig MP3 player with all the music I could possibly want? You bet, especially since I'll know the door is always open to download even more as my interests and tastes change.
Now stop and think about how this applies to the book publishing world. Could you imagine a model where you pay $X/month for access to an unlimited number of books? It's never going to happen in the print world but I think this could be the killer app for the Kindle, a world where manufacturing and distribution costs are zero. Access to every single book in the entire Kindle library could be yours for a monthly fee. Assuming the monthly fee is reasonable this could be the model that really kick-starts the e-book industry.
Full blog posting at Joe Wikert's Publishing 2020 Blog