I'm not an i-fan, nor do I own an iPad or iPhone, however, I have a friend that does and we've shared some titles.
If the books are downloaded from here, and don't have DRM, we found the simplest way was to email the book files; open the email on the iPhone, and download the file... your phone should find the app it needs to read the title (either iBooks or Kindle for iPhone app, depending on file type)...
HTH