I just purchased a book for my new NOOK HD, and accidently deleted it (I had downloaded the sample and was trying to delete the sample but ended up deleting the purchased book). I called customer support and they were very helpful. They pushed out another copy of the book to my device and all is well now.
That got me to thinking, as I accumulate books, what happens someday if the device fails, is stolen, broken, etc. Can I re-download all the books I have purchased on a replacement device? Is there a way to back them up? Maybe it is better to store books I'm not currently reading in the cloud?
I first tried to just go to the shop again, expecting it to show this was a book I had already purchased and thinking it would just allow me to download another copy. But this wasn't possible so I had to call customer support.
I have a NE1 Nook, a Simple Touch, a NC and a HD+. All my books are available on all my devices, non archived books downloaded automatically on all except the HD+, they are in the"cloud" and have to be individually downloaded. I have had to replace the NE1, and the NC and my books showed up on my replacement devices if they were not archived already but I have the choice to make them active for download.
Do be aware that if a book stops being carried at BN, it will no longer be available to you for download. You will then have to find it elsewhere and repurchase.
As a precaution, I download ALL of my books onto my PC via Nook for PC, import them into Calibre using my USB drive for the Library folder. So all my books are saved, backed up, My books are now safely stored in case my device or PC fails and I don't have to worry about books that are not available for download anymore. And I can always use calibre to upload those books to my device in the case that I want to read it, but it's no longer available.
I have not backed up my actual device as yet, but that should be a priority if you have a lot of enhanced books that may not be compatible with the Nook for PC or the Mac version. There are others here who can tell you what is best to use. Or you can do a search here using "backup nook" or "backup books" as keywords. that should yield that information for you.
There are several Nook apps available for non BN devices that will have access to your library as well. As long as BN continues to carry the book, it will be available to any nook device/app registered to your BN account.