I'd look up some of the free tools out there and backup/un-DRM your Amazon/Nook/Google e-books. There are some guides out there for extracting your books notes as well.
People are, for better or worse, entrusting more and more of their content to cloud services, things like photographs, documents they've created, music etc etc.
It's not renting really. If I rent a physical book (or house, etc), I have exclusive access to it. Nobody else can possess it. It's for this privilege that I'm paying. Obviously, this is not true of ebooks because 'X' persons can "rent" at the same time.
You're just licensing them. You're just paying for a license which can be revoked at Amazon's whim.