Take a look here https://discussbook.com/ and here view-source:https://discussbook.com/assets/plugin-third-party-6ae2c6dcefb0b7bf8b18b7b7f6c91a8882c9cde5f35e429cf3c2cb85fe77e855.js .
I think that site add a js script and a script src to include the API library Developer's Guide | Google Books APIs | Google for Developers