Interestingly, I had an email from Amazon the other day telling me that some of the CDs I have bought from them in the past (going back to 2005) are now available for me to download free as MP3 files. As it happened, of the three mentioned, I'd already converted two to MP3 files, but I was very happy to be able to download the third.
This is Amazon saying: Hey, you bought the CD, here, have the MP3 as well.
Maybe this really will happen with books and Kindles.