You can download any books that are from Barnes & Noble or that use the Adobe Digital Edition software onto your Nook. Nooks also can read pdf files.
Adobe Digital Editions is free software for your computer. You will need to create a free Adobe account to use it.
Downloading a pdf file to a Nook; Method 2
Downloading Ebsco E-books to a Nook
Downloading a CRCnetBASE book to a Nook
Downloading a National Academies Press book to a Nook
Downloading a Project Gutenberg book to a Nook