In order to read Adobe ePub and PDF ebooks bought from most sites (but not from B&N), you need to download the ebook using Windows or Mac Adobe Digital Editions. Then you can "side load" the ebook onto your Libre (which already includes its own version of mobile Adobe Digital Editions). For this to work you need to authorize the Libre one time. See
Books on Board: EZ Reader Help for screenshots of how to do this. All devices with mobile Adobe Digital Editions work the same way, so this is equally applicable to your Libre as to an EZ Reader.
As I said above, B&N ebooks are almost always encrypted with a username/password based on your credit card details. The Libre can't decrypt B&N ebooks, but there are tools available to strip the DRM (encryption) on a PC. If the original B&N ebook was an ePub, then the DRM-free version will then work on your Libre. If it was in the eReader ebook format then there is an extra step to convert to ePub. If you don't want to fiddle with all this decryption and format conversion, just buy from any of the many stores selling Adobe ebooks. Books on Board is one and (as Desertway says) Kobo books is another.