You can use the following types of ebooks on your NOOK:
EPUB (including Adobe Digital Editions)
If you purchased books from Fictionwise or eReader.com, B&N converted all purchases and put them onto your NOOK device.
Can I Read Word Documents or TXT Files on My Samsung Galaxy Tab 4 NOOK?
If you want to read Word documents or TXT files on your NOOK Tablet and treat them as ebooks versus Word documents, you need to first convert them into EPUB files.
Calibre can convert ...