I have been looking over the material at www.archive.org
and much of the material in the universal library is NOT public domain in the U. S. Unfortunately only copyright actions from 1978 on are online and for the earlier ones you have to go in person to the copyright office in Washington DC!
However, for those books published between 1950 and 1964 which might be public domain you can check
to see if copyright was renewed.
My experience was that everything that really interested me had its copyright renewed.