There is no “international copyright law” that protects the copyright of a work globally. However, there are several international copyright agreements that deal with the protection of works and rights of content creators. The most important of these agreements is the Berne Convention, according to which, once a work is protected by one convention member then it is protected by all convention members.
Moreover, it states that the scope, exceptions, and limitations of copyright are based upon the laws of the country in which the copyrighted content is used. For example, if an international journal is copied in the United States, then The United States Copyright Law along with its exceptions and limitations as related to reproduction of content will be applied to its use. Detailed information can be found at International Copyright Relations of the United States and International Copyright Section of The Copyright Clearance Center’s Copyright Basics.
All works in the public domain can be used by anyone and for any purpose without having to obtain permission or giving credit or attribution to the creator of those works. Categories of works that are in the public domain:
Works that never had copyright protection
Works that are no longer protected by copyright
Works published in the United States prior to 1924 are in the public domain
Works created by the United States government
For more information, see the Copyright Status Chart by the Cornell Copyright Information Center.
As a general rule, works published in the United States enjoy copyright protection until 70 years after the death of the author, even if those works have become out-of-print. Out of print simply means a book is not currently being published. It may or may not be published again in the future. The copyright holder still retains all rights whether a book is in print or not. Copying an out-of-print book without permission of the copyright holder is illegal under United States copyright law.