I have an interactive book that I have exported from iBooks Author to a .ibooks file. I seem to remember that one used to be able to sideload a non-drm iBooks file by opening the link in Safari on the iPad and telling it to open the file in iBooks. Apple's terms and conditions even allows you to distribute a .ibooks file via a web link as long as you are not charging for the content.
Using iOS 8 I've only been able to get Safari to prompt me for what to do with the file one time. Every subsequent attempt just ends with a blank white screen.
Using iOS 8 I've only been able to get Safari to prompt me for what to do with the file one time. Every subsequent attempt just ends with a blank white screen.