If your downloaded book isn't opening, first check your internet connection and ensure the book is fully downloaded; then, try re-downloading the book, as it might be a corrupted file; if that doesn't work, check your reading app settings, update the app to the latest version, and make sure you're signed in with the correct account used to purchase the book.

Ensure your reading app supports the file format of the downloaded book (e.g., EPUB, PDF).

GOOD LUCK,!!