Which version?

For Kindle and Kindle apps

.prc or .mobi

(email to your Kindle/Kinde app -- find address in settings -- or transfer via USB)

For Nook, Nook apps and most tablet apps

.epub

(transfer via USB or from cloud storage -- DropBox, Google Drive, Copy, etc -- used "Open In" to select the app on your tablet)

Desktop or laptop computers

.pdf or .epub

(download Adobe Digital Editions to read .epub files)