Oh, Hello, 80 Percent Of Books Published Between 1924 And 1963 Are In The Public Domain

Wow, thanks for your data mining project, New York Public Library! Why, though? Because “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.” And most people – and publishers – did not renew that registration. – Boing Boing