For Nook Readers: You can read Libby books in EPUB or PDF format only - Do not select the Kindle option.
To download a Libby eBook to your Nook device, please follow the instructions on the Barnes & Noble Nook help pages, Here.
You will need to use a computer, smart phone, or tablet to checkout the ebook from Libby, and then you can download the ebook to your device via a USB connection.