I found an interesting addition to this on Wikipedia (though I would not recommend it as a source for legal advice in general):
"Works published with notice of copyright or registered in unpublished form on or after January 1, 1923, and prior to January 1, 1964, had to be renewed during the 28th year of their first term of copyright to maintain copyright for a full 95-year term. With the exception of maps, music, and movies, the vast majority of works published in the United States before 1964 [my emphasis added] were never renewed for a second copyright term."
Included in this area is some truly great work such as H. Beam Piper's space opera classic Space Viking. All the more reason to help get these books back into people's hands.
More details and follow-up on developments with the Pulp Fantasy Society to follow later today.