Almost any book published before 1923 is currently in the public domain. But for books published in 1923 and after, copyright status is determined by a number of factors.
A book’s copyright status can get confusing very quickly, but fortunately the Samuelson Law, Technology & Public Policy Center at Berkeley Law has created a handbook to help determine whether a specific work is in the public domain. Posted below is the master flowchart from the handbook, and if you need more information check out the “Is It in the Public Domain?” [PDF] handbook at the Berkeley Law website. In addition to additional flowcharts, the handbook also includes just about anything you’d ever need to know about copyright status.
Click the flowchart for a full-size version of the image!