I tried looking through the AppData\Roaming\Barnes & Noble\BNDestopReader folder and subfolders as suggested in Alf's help file and could find nothing resembling the text file he describes with the encryption key. All I found was a ClientAPI.db and a folder CachedImages with all my book covers.
You need Nook Study, -and- you need to login using it and to download a book. It does not appear at this time that the book needs to be a newly purchased book, I believe that even archiving and re-downloading a book would generate the key needed.
Yes, once I loaded Nook study instead of Nook for PC I found the encryption key easily. So I can continue to future-proof all my purchases.
As much as B&N's library management and DRM annoy me, their reader app does a nice job with ebook formatting. My regular workflow is to prep my ebooks in Calibre, then download them to my tablet using Calibre Companion (CC). I use CC on my tablet to view my library, sort and search. From CC, when I select read, I'm presented with a list of Android reading apps. If I select NOOK, the ebooks opens up just fine in the B&N reader no matter where the file is stored. There's no multi-device sync, and the NOOK library ignores my sideloaded books, but the reader is nice. Unfortunately, the Current Read and Library widgets only show B&N library items.