To buy a Kindle book and read it on your iOS device, you can’t use an in-app bookstore. Instead, you have to go to Amazon’s website to buy your book. The same holds true for virtually every other iOS e-book reader. Yesterday, however, Apple rejected Sony’s e-reader app for the iPhone, arguing that apps that offer users to buy content outside of the app also have to make their virtual goods available through in-app purchases (read: purchases that allow Apple to take its 30% cut)