One advantage of Kindle and Nook, though. Their libraries are available on all devices. I can read my Nook books on my iPad, iPhone, Mac, Nook, or whatever. My library is in the cloud. iBooks doesn't even have an app on the Mac. (Why not?) Also, if I purchase a book on my iPad it does not show up in my library on my iPhone. The book only exists on the iPad. I have always wondered why Apple did it this way, given the iCloud thing. But that's how it works.
My favorite book of all time, btw, is Cryptonomicon by Neal Stephenson which I have as a Nook book. I have read it many times. It is awesome!