That is my whole complaint about books on the iOS device is that I cannot use them on any other device. I can play my music, movies, etc, anywhere else, but cannot read a book? How stupid is that???
Apple is losing out on a big share of the market by locking out that part of the software. I will find other ways of obtaining books in the future that I can read on my iPad. If I have to buy the Kindle or similar app for iPad just so I can read the same book on my PC, then so be it. Kindle will get the market commission and Apple will not. It's that simple.
And if it gets too difficult to cross platform a book to read and I am forced to get another pad device just to be able to read the same book on the airplane as well as on my PC at work during lunch break, then the iPad gets chucked for an Android device. I hold no loyalty to Apple or Android, I just want it to work!