System design for consuming third party APIs

I have a CRM system. I need to implement grabbing Contacts’/Accounts’ data from third party APIs by user provided IDs and loading details into CRM. The problem is that API responses may differ e.g. by Accounts’ legal form or something like that. Moreover I need to check received responses against other corporate internal information systems. I understand OOP and all that stuff rather well. But everything still looks very tangled and tedious. And I wonder if there are any methodology(ies) of working with external data. Any books to read or videos to watch? Any well-known design patterns for such tasks? Any ideas appreciated!