The public domain refers to works that are either no longer protected by copyright or they were never eligible to be protected by copyright. Since there is no copyright protection, the works can be used, distributed, reproduced, etc. freely, without permission. Books that were published in the United States before 1923 are in the public domain.
Since copyright law has changed over the years, there are works as recently published as 1989 that may have entered into the public domain because the author did not fulfill all of copyright requirements. To find out when works pass into the public domain, take a look at Cornell University's helpful PDF Copyright Term and the Public Domain. In order to determine if a work post-1923 is in the public domain, a fair amount of research will need to be conducted. The U.S. Copyright Office has created a helpful pamplet on How to Investigate the Copyright Status of a Work.