Just use something like Calibre to back them up and break the DRM on them (for the ones you can). I've had Amazon remove a few titles that I couldn't down load

Just use something like Calibre to back them up and break the DRM on them (for the ones you can). I've had Amazon remove a few titles that I couldn't down load later because of publisher disputes, and there have been many instances of them updating books for modern audiences that brought unwanted changes. The downside to always connected digital media - it can be ephemeral. In that regard books are far superior.