Here is a very nice table listing pretty much every case under US copyright law [1].
For books first published in the US, here is what is now in the public domain.
• Books published before 1924.
• Books published from 1924 through 1977 without a copyright notice.
• Books published from 1978 to early 1989 without a copyright notice and without a subsequent registration within 5 years.
• Books published from 1924 through 1963 with copyright notice but whose copyright was not renewed. (This is the case for the books the article is talking about).
• Books prepared by an officer or employee of the US government as part of the person's official duties.
For books first published in the US, here is what is now in the public domain.
• Books published before 1924.
• Books published from 1924 through 1977 without a copyright notice.
• Books published from 1978 to early 1989 without a copyright notice and without a subsequent registration within 5 years.
• Books published from 1924 through 1963 with copyright notice but whose copyright was not renewed. (This is the case for the books the article is talking about).
• Books prepared by an officer or employee of the US government as part of the person's official duties.
[1] https://copyright.cornell.edu/publicdomain