If you want to archive a book, you need to get it out of the Cloud and onto your personal computer!
I recently made my book exclusive to Amazon for 3 months. I was shocked to find that people who had purchased it through B&N could not re-download it from their library. Apparently, when an author withdraws a book from B&N, it becomes inaccessible to those who've already bought it. If they haven't downloaded it yet, too bad (although they can get a refund).
My book, Risen, is back at B&N at least through June 2012. It really seems unfair to me that someone can buy an ebook on the promise that it will be available through download, and then have that ebook vanish just because the author no longer sells through B&N. Once you buy it, you should have access to it.
So, I highly recommend the free ebook library management program Calibre. It's a tiny bit more trouble to move your books over to Calibre, but at least you'll have them when you want to upgrade your Nook or if you ever have to replace it.
I really hate all of this DRM/walled garden nonsense.
Author ofRisen, a supernatural thriller. Available DRM-free!