How do I backup my books?
Download Nook for PC/Mac or Nookstudy to your computer and then download all your books from there. Because I'm paranoid, I also make a copy in a separate folder.
You can also download them directly from the B&N website, but that takes quite a bit longer.
I've used Calibre eBook Management (http://calibre-ebook.com/) since I got my original Nook back in 2009 and it is a fabulous tool.
When you put your eBooks into Calibre, they can be found in the tool's 'library' on your PC.