You still buy the books through Amazon, and download them from Amazon. Technically they're not an IAP, since with "traditional" IAPs the app store/play store wo

You still buy the books through Amazon, and download them from Amazon. Technically they're not an IAP, since with "traditional" IAPs the app store/play store would keep track of what purchases a user has made; with Kindle, Amazon is keeping track of what you buy and they link it to your account so you can access it anywhere.