*Amazon is closing the Kindle digital book purchase * on the platform after removing all audio books from the play store. This is attributed to the new round of activities of Google play. Since June 1 this year, Google has required all play store applications to use Google play billing for digital purchase, otherwise they will be deleted from the market.
Now when you visit Amazon's app, you can still buy physical books, but the digital purchase shows a "why can't I buy on the app?" Link to. Amazon's link shows a pop-up window that reads: "in order to comply with the policy of the Google game store, you will no longer be able to buy new content from this app. You can create a reading list on the app and buy it through the browser on the [Amazon ] website".
The purchase of Amazon music has also been closed on the Google play app. This makes Amazon's Google play app consistent with IOS app, which also does not allow digital purchase. On Android, Amazon is pushing users to the website. They can still buy digital content or sign up for an unlimited subscription, which avoids the purchase lock of the play store.