Data-mining reveals that 80% of books published 1924-63 never had their copyrights renewed and are now in the public domain

Until the 1976 Copyright Act, US works were not copyrighted unless they were registered, and then they quickly became public domain unless that registration was renewed. The problem has been to figure out which of these works were in the public domain.

Source: Data-mining reveals that 80% of books published 1924-63 never had their copyrights renewed and are now in the public domain