I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
This article is about U. S. copyright law 1924 – 1963 books published in the U. S. where copyright was not renewed – Millions of Books Are Secretly in the Public Domain. You Can Download Them Free: https://www.vice.com/en_us/article/kz4e3e/millions-of-books-are-secretly-in-the-public-domain-you-can-download-them-free.
Prior to 1964, books had a 28-year copyright term. Extending it required authors or publishers to send in a separate form, and lots of people didn’t end up doing that. Thanks to the efforts of the New York Public Library, many of those public domain books are now free online. Through the 1970s, the Library of Congress published the Catalog of Copyright Entries, all the registration and renewals of America’s books. The Internet Archive has digital copies of these, but computers couldn’t read all the information and figuring out which books were public domain, and thus could be uploaded legally, was tedious. The actual, extremely convoluted specifics of why these books are in the public domain are detailed in a post by the New York Public Library*, which recently paid to parse the information in the Catalog of Copyright Entries.
*This article details a lot of the 1923 – 1964 changes in U. S. copyright law. As of January 1, 2019, 1923 published U. S. works became public domain.
If the book was never published, it falls under the law that took effect in 1978 which made every unpublished book, photograph, etc. subject to copyright. For books and other items published between 1924 and 1963 that were not copyright renewed, the copyright laws in effect at the time the work was first published or at the time the copyright ended determine how long copyright lasted. There were a number of copyright law changes during this period, not counting numerous changes made over later decades.
It’s worth noting you could have a book published in the U. S. and another country where the book is still under copyright in one country, but in the public domain in the other country.