Public domain refers to creative materials such as patent drawings that are not protected by copyright, trademark, patent, or any other intellectual property laws. The creative work is shared by the public and cannot be owned or protected by copyright laws.
As of January 1, 2019 work created before 1924 is now in the public domain.
Definition from Sanford University Libraries
It's been 20 years since the last big group of works have entered the Pulic Domain. Want to read a little more check out this article.
Will help determine if a book is in the public domain and can go online. There are links with where to get help to obtain permissions, copyright durations in other countries, and links to books in the public domain and online.
A link to Cornell University information on Copyright Terms in the United States.
http://Copyright Term and the Public Domain in the United States
This public domain slider will help determine if works are in the public domain for anyone ot use freely. Provided by the ALA may need a flash player to use.
There are four common ways that works arrive in the public domain:
https://fairuse.stanford.edu/overview/public-domain/welcome/
Image flickr Alice Popkorn 
he sites below will guide you to a cultural wealth of public domain books, images, illustrations, audio, and films where the copyright term has expired or the creator has not renewed the license. Remember, public domain works are free and available for unrestricted use. Enjoy and be creative!
University of Montana. Public Domain and Creative Commons: A Guide to Works You Can Use Freely
Ithaca College, Public Domain Material: Images, Audio, Scores & Books
This Public Domain Shepra Copyright Calculator requires a flash player. Answer the questions to help determine if a work is in the public domain yet. It only cover U.S, Copyright Law and public domain may vary in different countries.
