title_body: How to send library ebooks from Calibre to Nook
upvoted_answer: First, I assume that you are referring to "protected ebooks" which when checked out requires some kind of Digital Rights Management. If they are not protected ebooks, I imagine you could just transfer them by USB -- and wouldn't need to answer the question. 
  
  Second, I am assuming that you live inside the USA. 
  
  Does your library use the Overdrive app? (That is probably the most common ebook distributor for libraries). 
  
  If yes, and if you use one of the color Nooks (like NOOK HD, NOOK HD+, etc), you can download an Overdrive app onto your Nook http://company.overdrive.com/overdrive-app-for-nook-makes-library-ebook-borrowing-even-easier/
  
  It is much easier to check out ebooks via the Overdrive app than to transfer the encrypted epub file to your Nook via USB. 
  
  On a related note, I think it's worth upgrading to a Kindle Fire or an android tablet or even one of the Kindle e-ink devices -- especially if you use Overdrive. Even if your library uses another service beside Overdrive, chances are that they have a reading app which you can download to make it easier to check ebooks out directly. Nook is probably behind on connecting with libraries. 
  
  Did you know btw, that some libraries let you connect your Kindle e-ink device directly to your library, so if you check out the ebook, Overdrive (or another distributor) will send it directly to your e-ink device and then manage the checkin as well. Kindle is light years ahead of Nook. 
  
  Just recently Overdrive released a much more user friendly app called Libby which is much more useful than the previous Overdrive. It is available as an app in the Apple, Android and MS store. https://meet.libbyapp.com/