Here's a (not fully complete) list: https://fundamittere.de/literatur/index-en.html

Welcome from Pennsylvania!

Is there a particular topic about slinging that you're looking for?  There's books on making them, although those are mostly about braiding.  There are some about history.  Slings in warfare, finding of sling stones, etc.  There's not too many about use.  That Cliff Savage book is the only one that really comes to mind which hits on that kind of thing.  But videos are infinitely better for learning anyway.