A public-domain book is a book with no copyright, a book that was created without a license, or a book where its copyrights expired or have been forfeited.
In most countries the of copyright expires on the first day of January, 70 years after the death of the latest living author. The longest copyright term is in Mexico, which has life plus 100 years for all deaths since July 1928.
A notable exception is the United States, where every book and tale published before 1926 is in the public domain; American copyrights last for 95 years for books originally published between 1925 and 1978 if the copyright was properly registered and maintained.
Radio shows created before January 1, 1978 are protected by the Copyright Act of 1909 rather than the Copyright Act of 1976 because according to case law any copyright determinations must be made according the copyright law as it existed before that date. Assuming the old time radio shows were in the public domain from from the Copyright Act of 1909, the update of 1976 could not suddenly place them under copyright because they were already in the public domain, and the status of a public domain work is not allowed to ever be reversed.