I basically roll my eyes when folks say,
“No, really, that can’t happen with Kindle. It’s totally secure.”
You just do what I do, have a paper backup copy of the really "good" ones. Although I'm backing up hardbacks with paperbacks, although when I buy the paperback, the hardback is what goes on my shelves.
I had some time at the end of the day yesterday at work and if you google (something like), “kindle, hack, copy files”, you’ll find what I found. I wouldn’t use it for anything I was interested in keeping somewhat control over.