You were lucky. Most e-books from Amazon come with DRM and restrictions preventing you from reading them on devices that do not have a kindle app. I once purchased a textbook (that wasn't on libgen) on Amazon and had to use the Kindle app on Windows to read it. It was not readable on Kindle Cloud Reader and Amazon does not have a Kindle app for Linux. I returned the book after using it (only needed it for a week) and got a refund.