You can't buy books via Kindle Android app, but there are a few solutions
Amazon Kindle Buying Guide 2022 | The Strategist
Amazon Official Site: Kindle Paperwhite
Amazon Kindle Fire HDX 7" 16GB Black B00BWYQ9YE - Best Buy
Amazon Official Site: Kindle Paperwhite
Amazon.com: Amazon Kindle Paperwhite 11th Generation User Guide: Learn How to Use Your Kindle Paperwhite E-Reader. Set-Up, Navigate, Manage, and Master the device Like a Pro: Beginner Friendly, With Advance Tips eBook :
Amazon Kindle Paperwhite 11th Gen 8GB, Wi-Fi, 6.8" - Black (Ad-Supported) for sale online | eBay
Best Prime Day Kindle Deals for October 2022 | Digital Trends
Amazon.com: AMAZON KINDLE FIRE HD 8 (2020): The Complete User Guide to Master the Latest Kindle Fire HD 8 (10th Generation), With Tips and Tricks to operate and navigate Your Tablet eBook :
Amazon Kindle Fire HDX 8.9 (3rd Generation) 32GB, Wi-Fi, 8.9in - Black 848719015669 | eBay
Amazon Kindle Paperwhite 8GB Waterproof Ad-Supported 2017 Black B07CXG6C9W - Best Buy
How to Use a Kindle Without an Amazon Account - Make Tech Easier
Amazon Kindle Paperwhite 8GB Waterproof Ad-Supported 2019 Twilight Blue B07PS737QQ - Best Buy
Best Amazon Kindle 2022: Which One Should You Buy?