Why the Kindle is No iPod [2007]

I'm particularly interested in this claim:

"For the Kindle to be the "iPod of reading," it would have had to have been preceded by a "Napster of reading." And, of course, it wasn't. Which probably tells you something about the difference between songs and stories."

Is there anything intrinsic about books that makes a "napster of reading" impossible? The only reason I can think of is that reading a book is harder than listening to an album.