For at least the last year or so, I've been using the NOOK app for Android. I use
ADB to retrieve the books and then import them into Calibre. Recent versions of the app require you to have root access to retrieve the books.
If you don't have a rooted Android device, the NOOK app runs fine on
Android x86, either in a
VirtualBox virtual machine or booted from a USB stick.
I have version 7.1-r1 installed to a 16GB USB 3.0 thumb drive, but I've found that the older
version 5.1-rc1 runs better in a virtual machine.
Getting it up and running is pretty straightforward, but for me there was some trial-and-error involved. I can post instructions for getting it working, but if nobody else is interested in this particular method, I won't bother.