Suppose you wanted to assign Herman Pritchett's The Roosevelt Court (1948) in your class. It's out of print. Can you just
pirate copy the whole book? In answering this question, a good place to start is the recently updated Cornell Copyright Duration Chart.
There are a couple of simple rules: (1) works published before 1923 are in the public domain, but (2) works published by Disney are not and never will be. For all other works, see the chart.