This week's Free Fridays, The Emperor's Knife, is published by Night Shade Books. Night Shade is in the process of shutting down, which means that you cannot rely upon B&N's "virtual bookshelf" to keep copies for you. If you "buy" this title, download it immediately and make backups on your computer.
The same applies to any other Night Shade titles that you might have bought, whether at B&N or somewhere else. Be sure you have them backed up if you want to keep them.
Also, if there are any Night Shade titles that you've had on your e-book wish list, buy them now or you'll probably miss your chance. Some of those titles might eventually show up again under other publishers — Night Shade is trying to get authors to approve transfer of their e-book contracts to Start Publishing — but there will be questions of "if" and "when."
I'll also note that there's no guarantee that author royalties will be paid on any e-books you buy from Night Shade. Maybe they will, maybe they won't.
What process do you use to backup for nook books?