There were literally thousands of works that were published in the United States before 1964 which fell into the public domain. The copyright was not renewed and that means they are now public domain works.
It something was published before 1964, the owner had to file to renew the copyright before the 28th year of being published. If they failed to do so, they lost the copyright.
If you decide to use a published book written before 1964, you're going to need to do research to make certain it is legal for you to copy. If it was published before 1924, you can assume the works is in the public domain.
