It seems to me that DRM is at the bottom (or top) of all.
Kill DRM (
) and it doesn't matter if it's ePub, PDF, AZW or whatever as you may convert them as you like.
Personally i prefer ePub because of libraries and the ability to switch between devices. ePub is common in europe and there are even shops who sell it without DRM.
Isn't there a Qt enhancement for Kindle which enables you to read ePub ?
I believe i've read something i the Kindle area.