There are a lot of books – great ones at that – in public domain. You can find them at Project Gutenberg or Google Books. But how do you know if their copyright has expired or not? Well, here is a detailed and informative answer from a person who wanted to publish a book from early 1900s.
This is, of course, a very late answer, but it may help other people who stumble across this question. Any work that was printed before January 1, 1923 is in the public domain. January 1, 1923 is the cutoff for the first change that took place in copyright law. Any work before this date could only be copyrighted for a maximum of 56 years. Works printed from 1923 to 1963 only fall under the extended copyright laws if their copyright was renewed for a second 28 year term. Works printed before 1978 that were printed without a valid copyright notice are also in the public domain.
This is how this needlessly complicated law is written. Because of loopholes that isn’t always the way it works. The only way to be safe with any work printed after December 31, 1922 is to perform a copyright search. Otherwise you can get into a lot of trouble. The only safe area is on works printed before January 1, 1923.
You could therefore publish books that can help people out if there usefulness is still there.