Thanks to Illuminated Heritage for pointing out that Stanford has placed online a Copyright Renewal Database. Included are books published in the U. S. between 1923 and 1963 whose copyrights were renewed between 1950 and 1993 (records since 1978 were already searchable).
Anyone who transcribes data to place online should be aware that anything published before 1923 is fair game, but works from 1923 onwards may still be protected. Here's a snippet from elsewhere on Stanford's excellent copyright site:
Thousands of works published in the United States before 1964 fell into the public domain because the copyright was not timely renewed under the law in effect at that time. If a work was first published before 1964, the owner had to file a renewal with the Copyright Office during the 28th year after publication. No renewal meant a loss of copyright. [Link]If a work does appear in the database, that means it is protected for an additional 95 years from the date of renewal. A quick check of the database confirms what common sense would suggest: most family histories and locally-produced town histories published before 1964 are in the public domain, and now belong to us all.