Kudos to the New York Public Library (#NYPL) for mining this vein of gold beneath our feet.
https://www.vice.com/en/article/epzyde/librarians-are-finding-thousands-of-books-no-longer-protected-by-copyright-law
"NYPL has been reviewing the US #Copyright Office’s official…records for [books] whose copyrights haven’t been renewed…The books in question were published between 1923 & 1964, before changes to US copyright law removed the requirement for rights holders to renew their copyrights…Around 65 -75% of rights holders opted not to renew their copyrights."