You need to make a .env file in the same directory as this file. It consists of the following environment variable-

API_KEY = <your google api key>

You need to get valid API key for Books API by following steps -

Create a New Project: Click on the project drop-down on the top-left. Click on "New Project," name it (e.g., "Book Search"), and click "Create."

In the Google Cloud Console, navigate to APIs & Services > Library. Search for "Google Books API" and click on it. Click the "Enable" button.

Navigate to APIs & Services > Credentials. Click on "Create Credentials" and select "API Key."

Your new API key will be generated. Copy it and put it in the .env file.