Interesting research:

But even this chart may understate the effects of copyright, since the comparison assumes that the same quantity of books has been published every decade. This is of course not the case: Increasing literacy coupled with technological efficiencies mean that far more titles are published per year in the 21st century than in the 19th. The exact number per year for the last 200 years is unknown, but Heald and his assistants were able to arrive at a pretty good approximation by relying on the number of titles available for each year in WorldCat, a library catalog that contains the complete listings of 72,000 libraries around the world. He then normalized his graph to the decade of the 1990s, which saw the greatest number of titles published. By this calculation, the effect of copyright appears extreme.

Heald says that the WorldCat research showed, for example, that there were eight times as many books published in the 1980s as in the 1880s, but there are roughly as many titles available on Amazon for the two decades. 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.

via The Hole in Our Collective Memory: How Copyright Made Mid-Century Books Vanish – Rebecca J. Rosen – The Atlantic.