You can't buy books via Kindle Android app, but there are a few solutions
Amazon Kindle - Apps on Google Play
Amazon overhauls Kindle app with new look and deep Goodreads social integration – GeekWire
Amazon Kindle - Apps on Google Play
Kindle App
Amazon.co.uk: Free Kindle App: Kindle Store
 "Amazon to no longer offer in-app Kindle book purchases on Android | Technology News - Business Standard")
Amazon to no longer offer in-app Kindle book purchases on Android | Technology News - Business Standard
Kindle for Android:Amazon.co.uk:Appstore for Android
Changes to Android Kindle Policy - what does it mean? : r/kindle
Kindle for Android:Amazon.co.uk:Appstore for Android
Amazon Kindle for Android Version 8.58 Disables the Store - Good e-Reader
Kindle for Android:Amazon.co.uk:Appstore for Android
Free Kindle Reading Apps for iOS, Android, Mac, and PC
In The Kindle App For Android, Amazon Has Disabled E-Book Purchases
Kindle for Android:Amazon.co.uk:Appstore for Android
A Look at Amazon's Redesigned Kindle App for iOS and Android
Kindle for Android:Amazon.com:Appstore for Android
You can no longer buy Kindle eBooks on Android - Good e-Reader
Kindle for Android:Amazon.co.uk:Appstore for Android
Kindle for Android:Amazon.co.uk:Appstore for Android
Amazon Kindle on the App Store
Kindle app for Android now offers split-screen support, new 'Notification Center' incoming
How To Run Free Android Apps On the Kindle Fire | PCMag
