Re: Amazon/DRM/backup files.
When you buy a kindle book from Amazon, you agree to their terms of service which says you can't copy/resell the book. If there's DRM, you basically agree that you won't remove it. For any reason. MANY people feel that's an unreasonable clause and have no moral problem doing so when their intention is to use it on their own devices and they will NOT be sharing it with others or trying to re-sell it or anything else.
The clause in the ToS is in there because big publishers seem to feel that, without it, the books will be pirated and they'll lose money. The flaw in the logic is that it's easy to remove the DRM and pirate the books if one so chooses. So it's not really solving the problem they think they have. I don't think it hugely inconveniences most customers either, mind you; I just think it's not really necessary.
So . . . again, many people go ahead and make a DRM free copy of every book they buy on Amazon. That's their choice, and if Amazon goes belly up, they will for sure still have access to all their books.
I'd go so far as to say that even a DRM'd back up is sufficient. For now, you can still re-download to new devices directly from Amazon. If Amazon fails, then any agreement you had with Amazon about NOT removing DRM, it seems to me, would be null and void. So if you have one copy of the book for Kindle A, at such time as Amazon is no longer operating
you'd have a right to remove DRM from it to read it freely on devices B, C, D etc. And I'd not have a moral problem with that either.
For me, I don't worry about it with Amazon purchases. For one thing, there are very few books I would be likely to re-read; I spent what I spent and I got the enjoyment out of it. With paper books I didn't re-read either and most years donated several boxes to charities.
Secondly, I think the chances of Amazon being gone any time soon is essentially zero. That's my opinion and anyone may choose to disagree. Or agree but take precautions anyway.
I do, however, take the news discussed in this thread as cautionary. If I were inclined to buy from other vendors, ones not as large as amazon, it seems clear that the possibility of them closing nearly without warning is absolutely significant. It may even be probable. So I'd for sure make back up copies in the event that happened and, again, would have no qualms about removing DRM as necessary to still have access to them.