While you still could transfer library audiobooks and ebooks to the Nook, the easiest trick is to use OverDrive's own Android app to find and download the content. This is the singlt most widely used library ebook distributor, and they have over 19 thousand partner libraries around the world. That's why I have been wanting, for nearly a year now, to see this app available in the Nook App Store.
And today it is. I just got a Facebook update from a B&N insider with a link to the new listing in the Nook App Store.
I have not tested it yet, but it should work just like the OD Android app. You can search for libraries, find content you want to checkout, log-in, and download it - all from inside the app.