Public domain books are volumes that have no associated copyright and may be used freely by anyone. A book may become public domain if its copyright is not renewed or is released by the author. The time that needs to pass for a previously copyrighted book to enter the public domain depends on the statutes in the particular country. Other books are declared as public domain immediately after they are created, sometimes by using some form of copyleft license. Many public domain books are available through Internet resources since they are free to reproduce and distribute.
Copyright law exists to protect the interests of authors and other content creators or publishers. Each country has its own unique copyright laws that determine how long these rights last, though international agreements such as the Berne Convention and the World Intellectual Property Organization (WIPO) copyright treaty have standardized some aspects. Copyright typically lasts for the life of the author plus an additional span during which his heirs can control the works. Some countries also allow the copyright term to be extended if the heirs or estate applies for it, which can prevent books from entering the public domain.
After the proper amount of time has transpired, most copyrighted works become public domain books. This means that anyone is able to modify, publish, or distribute them. In some cases a government may make exceptions, which typically applies to a single book and the entity to which the author left his copyright claim. Works that are offered this protection by the local government will never become public domain books unless the law is changed. This relatively uncommon practice is known as perpetual copyright.
Other works are considered public domain books to begin with. In the United States and some other countries, anything created by the government immediately enters public domain. An example in the United States is building codes. This type of material can never be copyrighted even if it is included in a derivative work.
When an author creates a book he may immediately release it to the public domain. Organizations exist to promote this type of behavior, and a number of licenses have been created to allow an author to release some rights without giving up complete control. This is sometimes referred to as copyleft, a play on the term copyright. Some common copyleft licenses allow reproductions and derivatives of a work to be made as long as they also remain in the public domain.