A book published during the presidency of Chester A. Arthur has a greater chance of being in print today than one published during the time of Reagan.
This is not a gently sloping downward curve. Publishers seem unwilling to sell their books on Amazon for more than a few years after their initial publication. The data suggest that publishing business models make books disappear fairly shortly after their publication and long before they are scheduled to fall into the public domain. Copyright law then deters their reappearance as long as they are owned. On the left side of the graph before 1920, the decline presents a more gentle time-sensitive downward sloping curve.