"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.