The U.S. Copyright Office has not yet digitized their records prior to 1978, so there are various online “digital” versions of the catalog for pre-1978 works. The version linked below is published by the University of Pennsylvania. These records also include copyright renewals for works between 1923 and 1978, since former U.S. Law required renewals of copyrights in order to remain protected. There are thousands of works that were never renewed during that time period, and if they were not renewed they are now considered to be in the public domain for failure to comply with U.S. Copyright formalities. A famous example of a work that failed to renew its copyright is the classic film It’s A Wonderful Life, originally published in 1946.
Catalog of Copyright Entries - for researching copyright registration and renewals for works published between 1923 and 1978.