"Which books are in copyright, not being published, and contain unique valuable knowledge?" Most works in a large number of academic domains from the twentieth century.

In my own field (at the intersection of linguistics, history and archaeology) I am constantly, multiple times per day, referring to information in publications from e.g the 1960s or 1970s that never got republished later except as a brief citation to that old book. And guess what, nearly all that twentieth-century scholarship is still under copyright, often from the big German or Dutch publishers that enforce their claims fiercely. The academic community has been doing a lot of work to scan our institutional libraries and upload them to the shadow libraries, but this is still all illegal copyright violation.

You're going to make me quote the "Dive Manual" scene from _Cryptonomicon_ again, aren't you.

(It features a software engineer introspecting about the fact that his line of work has caused him to dramatically overvalue recency when evaluating books about... pretty much any other technical field.)