If you want to read the book on a Kindle device or Kindle App, choose the MOBI file and follow one of the methods outlined here to send it to your Kindle device or App. We personally use the “send to Kindle by email” option.
If you don’t use Kindle, then you want to choose from either ePUB or PDF. Many apps and devices are compatible with the ePUB and PDF format, including but not limited to: iBooks, Google Play Books, Nook, Kobo, etc. They can be opened in most web browsers and a variety of free apps.