The 2D histogram is very interesting, but difficult to interpret, because of the various samplings and normalizations that could be relevant: number of books pu

The 2D histogram is very interesting, but difficult to interpret, because of the various samplings and normalizations that could be relevant: number of books published; number (fraction) of books digitized. If I had to summarize the big picture, taking the contour at the top of the dark blue band: coverage was sporadic until ~1700; then more books were published, and the references gradually moved back to 1500 over the next 100 years, but did not consider the future very much. 1800 seems to be a singularity (but perhaps due to sampling effects), where interest in the deep past and the future exploded in a short period. There are also interesting lines in the plot. First the vertical lines. There seems to be one (or more) books in 1530 that discussed the far future up to 1900. Then another futurist work around 1610. There's a history book published ~1580, that covers then recent past, and another cluster around 1650, perhaps including Hobbes's 'Leviathan'. Of course we miss Gibbon's 'Decline and Fall' because it covers antiquity, off the chart. Secondly, the horizontal lines. There is a bright line at ~1100, perhaps the First Crusade, and more at ~1300, ~1600 ~1800. The line at ~1350 could be the Black Death. Finally, there are the shadows of the 20th century wars, when history was not so popular, because there were more pressing current events to be written about, and all the historians were fighting at the front.