To the OP, I am so glad you asked this question. I have been wondering about the other apps myself. Right now I used the Android Market, Google Books app. It was irritating to know that I had to check the MAKE AVAILABLE OFFLINE before losing my wifi. I bought 3 books while my ship was in port in Hawaii, and I didn't know I had to check that box. Now I have 3 books I can't read until I make my next port (I'm a Marine on a Navy ship, deployed)
I have yet to use the Amazon Kindle App, or the Barnes & Noble App. From what I have seen on the internet, most people say Amazon because it has a bigger library. That's all I know on the subject. Google Books, I can't say was good or bad with nothing to compare it against. I do wish it was common knowledge I had to make available offline....grrrrr.