Copyright is supposed to encourage the development of creative works, and short terms of copyright doubtless do. But today copyright lasts essentially forever, and that has made decades of books disappear:
A book published in 1922 is in the public domain today. A book published in 1923 (that observed the proper copyright formalities) is still copyrighted today, and probably will be forever. And, as the chart shows, some point during the 1920s is exactly when books disappear.
This is tragic. A tool that was supposed to enrich our culture has been perverted by the content owners and their cronies to impoverish our culture instead.