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 Bono"!

The copyright law originally stated that a copyright would last for 75 years. Thus, if a book were copyrighted in 1900, the copyright would expire in 1975. If a book were published in 1920, the copyright would expire in 1995. If a book were copyrighted in 1922, the copyright would expire in 1997. But ... if a book was copyrighted in 1923 or later, then things take on a whole new slant ... courtesy of then-Congressman Sonny Bono.

You see, a particular item was originally copyrighted in 1923. It's copyright was due to expire in 1998 -- sending this item into the public domain. This is normally not a problem -- truthfully, most people don't care about things copyrighted so far back. Expiring the copyright can actually grant these works new life in the public domain.

In this case, though, the item due to expire was a mouse -- Micky Mouse!

Disney was understandably very concerned about their flagship icon becoming available to anyone in the public domain. Congressman Sonny Bono introduced legislation to extend copyright protection for an additional 20 years. This legislation passed in 1998 -- but only applied to items that were not yet in the public domain. Thus, an item copyrighted in 1992 was public domain after 75 years -- but anything copyrighted in or after 1923 was protected for 95 years.

So -- to play it perfectly safe and legal -- if the copyright date on a published work is 1922 or earlier, the item is now in the public domain and may be freely used (but not necessarily re-copyrighted and sold). If the copyright date is 1923 or later, it's best to leave it alone till at least the year 2018!

So what good is a book published and copyrighted prior to 1923? Well, if it's a book of sheet music -- or a book of classic poetry, literature, or theater -- or a book of formula, travel, or ... well, you get the idea. Some things become dated -- but some things are just as current today as they were when they were originally published.

This doesn't mean you can find a book by Mark Twain in the bookstore and freely copy and distribute it. You'll likely find the copyright on that published work is fairly current. However, if you find an old version of the same book published and copyrighted prior to 1923, then you can safely use it!

The best places to spot these treasures -- try your local public library, used bookstores, or estate sales.