This is a fascinating Article, so read it all and the details matter. As a teaser:

According to Greg Cram, associate general counsel and director of information policy at [New York Public Library], an initial overview of books published in that period shows that around 65 to 75 percent of rights holders opted not to renew their copyrights. 

If this project can get a firm handle on what from 1923 and 1964 is in the public Domain, that will benefit all of us. A flood of new works would enter the public domain. Imagine the history and literature that would instantly be available. The result would be fantastic!