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.