There is now an excellent book on public domain material in the Internet age titled The Public Domain: How to Find and Use Copyright-Free Writings, Music, Art & More by Stephen Fishman. I found this book to be very enlightening on the topic of copyright and the nuances there can be.

Other related books on copyright and public domain issues:

Free Culture Amazon | Powells BookSense

Copyright Plain and Simple Amazon | Powells BookSense

Intellectual Property: Patents, Trademarks, and Copyright Amazon | Powells BookSense

The Illustrated Story of Copyright Amazon | Powells BookSense

The Future of Ideas Amazon | Powells BookSense

There are also many books containing public domain materials which you can use, including:

Copyrights do expire, and in the United States the basic rule is that books expire 75 years after publication. So any book published before 1923 is safe to consider as being in the public domain.