If you want to leave the DRM intact, then buy Adobe ePub or PDF for the Libre. These are available from many ebook stores, including the Sony ebook store, but not from B&N.
B&N uses either eReader or ePub with a password (credit card name and number). If you download within B&N's Windows Reader you might get ePub, but otherwise eReader is more likely. The DRM on either one is circumventable, see
How can I read Barns & Noble books on my Sony 700? (Sony's also read ePub) and
Barnes & Noble DRM for EPUB circumvented.