Can you provide an example of a URL you’ve used to get a JSON response from the Google Books API? A white screen usually indicates that you’re getting an error from the API server. But if you’re starting with a valid JSON response, we can probably help you parse the data you need. If you’re starting with an invalid JSON response (a 404 error, etc.) then that needs to get fixed before Thunkable is even a consideration.
You’re getting a white screen because you’re trying to retrieve a property that doesn’t exist (“item”). So you need an if/then to see if that property exists.
But the bigger issue — as you mentioned — is that the ISBN call isn’t returning the right information. That was the first time I’ve used Google Books API so I don’t know the answer to that.
I would focus on getting the url call to work in a browser before you do anything else in Thunkable.
I tested this using a different book with the same code on Thunkable and it worked. I think I will defiantly need to add code that checks if it returned a book before parsing before I update my currently published app to have this feature.