...
Use the reinforced library ISBN plus “PBK” as the proprietary ID for each paperback product format row in the “product_formats” ingestion template;
Use the reinforced library ISBN plus “PDF” as the proprietary ID for each PDF product format row in the “product_formats” ingestion template;
Use the reinforced library ISBN plus “EPB” as the proprietary ID for each ePub product format row in the “product_formats” ingestion template;
Etc…
There is nothing special about the suffixes that I am adding to the original ISBN. I recommend letters that abbreviate the format because on seeing the proprietary ID you can decipher certain information about the record it’s assigned to.
You could use numbers instead of letters and skip the repeating the original ISBN entirely but then you lose these insights.