Un-DRM'ing B&N books so that you can read them in Calibre/FBReader, I can say from personal experience, requires getting the decryption key by running an Android emulator, and then putting that into the Calibre plug-in. See https://www.aricrenzo.com/2019-12-13-Liberate-Your-Nook-Ebooks/
Re: Downloading and reading a Nook/epub ebook with FBReader and or Calibre on my laptop?
Un-DRM'ing B&N books so that you can read them in Calibre/FBReader, I can say from personal experience, requires getting the decryption key by running an Android emulator, and then putting that into the Calibre plug-in. See https://www.aricrenzo.com/2019-12-13-Liberate-Your-Nook-Ebooks/