I was avoiding mentioning this, not just because of the questionable legality in some counties, but rather because it's more of a workaround. All Amazon has to do is sufficiently alter the encryption mechanisms for new books.
Same thing goes with Spotify. Tools exist to strip the encryption from the offline cache (they are either encrypted mp3 or vorbis, don't remember), but it's a workaround. I suspect it's why they killed off libspotify, which sucked because I used that to make a hackjob spotify client for RPi...
Well my point is you can make that backup now for any books you already purchased and if they change it in the future you can decide to not buy any more books. Spotify doesn't let you buy any music so I have never wanted to strip its DRM and haven't even looked into how to do it.
