I don't use B&N, so I can only give you generic instructions.

In the Library app, change the top left dropdown to "Storage" and navigate into "Internal Storage". Look for the folder in which the B&N app keeps its books (EPUB files). When you find the files, tap-and-hold on a file and select "Open in Reader" from the wheel menu. Please let me know the name of the folder so I can update my ebook tips post

If cheyennedonna reads this post, perhaps she could respond, too