Retrieve the key (and the books, if you want) from the NOOK app for Android. I think that's the only way at the moment.
Brief instructions for retrieving the key from a rooted Android device are
here.
The
adb command line tool is installed as part of
Android Studio.
The
sqlite3 command line tool is
here.
If you don't have a rooted Android phone or tablet, you can run the NOOK app in Android Studio's AVD emulator/virtual machine.
Book files are stored in main storage at "/data/data/bn.ereader/files/B&N Downloads/Books" or on the SD Card at "
path_to_sdcard/Android/data/bn.ereader/files/Download".
path_to_sdcard varies slightly by Android version and device, but is often "/storage/sdcard1" or something similar.