Copyright is not permanent, and works do eventually enter the public domain, where can be reused freely, without any permission.  However, copyright term is often longer than expected, and changes in copyright law over time mean the length of copyright term may vary with the age of the work. 

Broadly speaking, works first published in the US before 1923 are in the public domain.  Also, US government works (though not state or local government works) never have copyright protection, and enter the public domain on publication.  Beyond those two categories, it can be challenging to determine the copyright status of a US work, and it is generally not safe to assume a work is in the public domain. 

By international treaty, we must respect the full copyright term of works in their home countries, and many countries have copyright terms that run for the life of the author plus 50 or 70 years.  Therefore, without researching the specific title, it is generally not safe to assume that an international work is in the public domain until it is at least 150 years old.