This is interesting: a cloud-based version of the Kindle app.  I’ve had a quick play with it and it works really well – it’s very fast and good-looking. The help pages say “Kindle Cloud Reader is compatible with PC Windows, or Mac, or Linux computers using the Google Chrome or Safari web browsers, on Linux computers using Google Chrome, and on an iPad using the Safari web browser.”

That’s one way around Apple’s ban on in-app purchases.