I offer both PDF and ePub versions of my iPad book called iPad Academy: Tips, Tricks & Clever Techniques. I found some folks were confused by which version they should order or whether they should get both. So I wrote an article explaining the PDF and ePub formats.
During my lunchtime reading today, I found an an article on the Edudemic website that briefly summarizes six popular digital book formats, including file formats and devices for displaying eBooks. If you’d like to learn a bit more about eBook file formats and readers, see this article.