Hey @simonp , interesting and a little complex question
Maybe someone else has another idea, but this is how you could solve this:
Change the webhook settings to enable “JSON passthrough”. This would send all the data as text within your trigger.
Use a RegEx module using the “Text Parser” → “Match Pattern” in make, and create a pattern to search for the ISBN code. The input data would be the JSON from the trigger.
After the regex is complete, it would find your ISBN code. This can now be used in the following modules.
Use the “JSON” module to parse the json text from step 1
Use a map() function and try to find the ISBN object underneath Data by searching for the ISBN code you extracted using the RegEx pattern
This maybe sounds like a lot of things, which it is actually, but once you do this a few times it gets easier
Hope it helps!