MTed, according to Lolly Gassaway's chart, U.S. works published before 1964 are now in the public domain if they were not renewed. If they were renewed, they are in the public domain only if they were published before 1923.
Finding out if a copyright was renewed it not easy, but some helpful links can be found here.
Lolly Gassaway's chart doesn't mention this specifically, but certain foreign works that lost copyright due to non-renewal were restored to copyright by a 1995 law called the URAA, which implements the GATT in U.S. domestic law. If these foreign works were published in 1923 or later, then, as I understand, their copyrights are good until at least 2019.