The source of that massive fall-off at the midpoint is seemingly simple: all books published in the U.S. in 1922 or earlier are in the public domain. What’s immediately apparent from this graph is the fact that copyright is limiting the public’s access to older works—but why and how, exactly? The answer lies in the reality of what a copyright is really worth, commercially, and how long it retains that value—and it sheds light on another problem with copyright law.
Pretty staggering graph. Though exact the culprit is really uncertainty over copyright status, rather than the length of copyrights per se.