Nook book: The EPUB file can be viewed on virtually any device and in many different ebook reading applications.
Kindle ebook: If you have Kindle for PC or Kindle for Mac installed on your computer, then double-clicking the .mobi file will open the book in one of those applications. You can also send the .mobi file to your Kindle by e-mail (see the user manual of your device). The Kindle reading applications will be easier on your eyes if you set the background color to “Sepia” and reduce brightness (use the “Aa” button to change the settings).