Heald says the picture is "quite dramatic." The most recent decade looks better by comparison, but the depression of the 20th century is still notable, followed by a boom for the most recent decades when works fall into the public domain. Presumably, Heald writes, in a market with no copyright distortion, these graphs would show "a fairly smoothly downward sloping curve from the decade 2000-2010 to the decade of 1800-1810 based on the assumption that works generally become less popular as they age (and therefore are less desirable to market)." But that's not at all what we see. "Instead," he continues, "the curve declines sharply and
quickly, and then rebounds significantly for books currently in the public domain initially published before 1923."
Heald's conclusion? Copyright "makes books disappear"; its expiration brings them back to life. Books which are most affected are those from recent decades, such as the 80s and 90s, for which there is a large gap between the abstract notion of people's interest and what is available. 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 current publishing business models makes books disappear 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 time-sensitive downward sloping curve. But, this chart may also understate the effects of copyright, since this comparison assumes the same quantity of books has been published each decade. This is, clearly, not the case. Increasing literacy coupled with technological efficiencies means far more titles are published per year in the 21st century than in the 19th.