I think the '80s are a bad example, for that reason, as a relatively recent decade. The '30s are much more problematic: the material is mostly still in copyrigh

I think the '80s are a bad example, for that reason, as a relatively recent decade. The '30s are much more problematic: the material is mostly still in copyright, but the original authors are rarely alive, and in many cases even close heirs (like their children) are no longer alive. It's common for works from that era end up being either orphan works with the copyright holder unfindable, or else owned by someone very distantly related to the person who wrote the book. The 1980s example is mostly interesting for seeing just how quickly almost everything drops out of print.