There are two main formats in use for eBooks. The first – MOBI – is used by Amazon and the second – EPUB – is used by pretty much everyone else.
I made Newton’s Ark available in EPUB format via Barnes and Noble, Apple and Sony using Smashwords as the intermediary. I haven’t been entirely satisfied with that arrangement for a number of reasons, the most important being that it gives me very little control over the information that appears in those online bookstores.
I’m going to try a different approach with Fullers Mine. If you want a copy in EPUB format to use with your Nook or Sony Reader or Apple’s iBooks app, just send proof that you purchased the paperback or Kindle version to firstname.lastname@example.org and I’ll send you the file.