One of the easiest places to find authoritative public domain content is ... your local used bookstore or library!True -- virtually everything contained in the books you'll find at these locations were protected by copyright. But ... it's also true that many of these copyrights have expired -- making the contents of those volumes completely public domain.US copyright law has gone through several iterations -- but the two things you need to remember are "75 years from copyright date" and "Sonny .