So... what I've always read is that a basic rule of thumb pretty much goes as follows: if any book was published in 1923 or earlier, it's public domain. HOWEVER, Circular 22: How to Investigate the Copyright Status of a Work (Copyright Office, Library of Congress, January, 1991) says that:
Going by the 75-year rule, anything published in 1937 or earlier would be public domain. But this doesn't seem as if it could possibly be right, because Gutenberg certainly doesn't go by that rule. Still... it's hard to see what else this could mean, because it does say that the copyright will expire 75 years from the end of the year in which it was FIRST secured-- not from when the copyright was renewed (if it was.) I am probably just missing something very obvious. Does someone know what the answer is here?WORKS PUBLISHED AND COPYRIGHTED BEFORE JANUARY 1, 1978: A work published before January 1, 1978, and copyrighted within the past 75 years may still be protected by copyright in the United States if a valid renewal registration was made during the 28th year of the first term of the copyright. If renewed, and if still valid under the other provisions of the law, the copyright will expire 75 years from the end of the year in which it was first secured. Therefore, the United States copyright in any work published or copyrighted more than 75 years ago (75 years from January 1st in the present year) has expired by operation of law, and the work has permanently fallen into the public domain in the United States. For example, on January 1, 1991, copyright in works first published or copyrighted before January 1, 1916, will have expired on January 1, 1992, copyright in works first published or copyrighted before January 1, 1917, will have expired.
BTW, the original link to Circular 22 is here.
ETA: I think I may have found the answer, and Sonny Bono is involved. Do let me know if that's it.