If you've upgraded your app, your books will continue to be stored in the same location they were before the upgrade.
The Google Play Books app automatically stores books in the location with the most free space, whether that's your device or your SD card, as of when the app was first launched. In addition, it sends you privacy alerts about apps that can get user permissions to access your personal information, violating the Developer Policy.I checked several sources on this.
It will warn you about detected apps that violate the Google Play Store unwanted software policy. Yes, Google Play Store is a safe app where the content is checked to ensure you don't take risks. Yes, Google Play Store is 100% free to download and use.
How to install Google Play Store on your PC? You can also navigate across the different sections, depending on whether you are looking for apps, games, books, etc. Then, you just have to type the names of the apps you want in the search bar and find what fits your needs. Once Google Play Store is downloaded, installed, and launched, you can log in with your account. They also provide tools and free training to help developers manage app releases, performance, and account and payment management to make sure you are used to the platform.