Stanford's Copyright Renewal Database is a searchable index of the copyright renewal records for books published in the US between 1923 and 1963. Note that the database includes only renewal records, not original registrations, and only Class A (book) renewals received by the US Copyright Office between 1950 and 1992.
The period from 1923-1963 is of special interest for US copyrights, as works published after January 1, 1964 had their copyrights automatically renewed by statute, and works published before 1923 have generally fallen into the public domain. Between those dates, determining whether a work's registration was renewed as required has been a challenge. Renewals received by the Copyright Office after 1977 are searchable in an online database, but renewals received between 1950 and 1977 were distributed only in a semi-annual print publication which has not been made searchable. The Copyright Renewal Database brings those records together in a searchable format.