When we say that something is in the "public domain" we mean that it is free for others to use without obtaining permission from the owner. In other words, it is no longer protected by copyright. Additionally items produced by the Federal Government are in the public domain.
In the United States, all materials published before 1928 are in the public domain as of 2023.* Each January 1, the literature, movies, music scores, and other works released 95 years earlier will enter the public domain.
So, if a user wants to make a copy of an entire book that was published in the United States in 1921, the answer is: go for it! That particular book is in the public domain.
One easy way to check if a particular item is, indeed, in the public domain (especially for books) is to check the HathiTrust database. If the entire item is publicly accessible in the HathiTrust database, the library user is free to download and print that item.
Also, referring the library user to the HathiTrust database is a nice way to save them the time, effort, and expense of creating a physical copy of the work. The entire book/work might be publicly accessible to them for free in the HathiTrust database, so please refer users to that source for works in the public domain in the United States (note that some international items may also be available in HathiTrust, too).
Note: International publications are different than United States publications and will be addressed later, but this can get complicated quickly, so feel free to refer such difficult questions to Sara Benson at: srbenson@illinois.edu
*Note that if a particular work has entered the public domain, but a new annotated version of the has been published since then, then the annotated version may not fall into the public domain.
The copyright status for works published in the United States between 1925 and 1968 varies depending on whether certain "formation" requirements were met. For instance, a work published in 1928 without a notice of copyright would fail to meet the legal requirements for copyright in that year and, as such, would be a public domain work.
Similarly, a work that contained a copyright notice, and was registered with the US Copyright office in 1928, but the copyright was never renewed in the 28th year after publication would also be a public domain work.
How do you know which "formation" (such as copyright notice and renewal) requirements were required in which year?
A few good sources include:
Let's say you've determined that the book was published in 1928 with the appropriate notice and registration. How can you tell whether the registration was renewed? Go to Stanford's handy copyright renewal database.