Downloading digital books can be tricky sometimes.
Epubs and Kindle (mobi) files can be downloaded. Bookfunnel is used to download your books. They will help you if you have any issues.
To download epub file to an ipad:
First of all, you need to have the ibooks app installed on your ipad. If you don't already have it, then you can go to the app store on your ipad and download it. It is free.