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.