I was wondering the same thing. I see a slight price difference in Canadian Dollars. Between the Android and iOS versions of the books.
Right now I think the best choice is to get the “web access” version if you don’t want to be confined to any platform but you need to have internet access whenever you want to access the books.
If you want offline access then it’s best to get any other version. If you have a Mac computer the iBook version would work on the Mac and all your iOS devices. If you have a chromebook my guess is the Android version would work on that but i’m not 100% sure.
Not sure if there are any differences between the app and book versions. I am still trying to figure that out.
@caviar dreams has written a good summary detailing the pros of the web and ios vs ibooks versions. Regarding slight price differences in currencies, we are aware there is the possibility for some currency arbitrage as sometimes there is a delay in updates in the stores as exchange rates fluctuate. There is no difference in content between the different versions of the book.