And Pray tell where were most documents/books printed during that era?"
The graph shows the use of "is" versus "are" began in the 1790s and reached a peak of 1/3 around 1820.
Do you remember what that era is noted for?
It was the "era of good feelings", President Monroe's second term, reelected without opposition, a time when Americans felt good about their country and were therefore more likely to think of it as singular rather than plural.
That "era of good feelings" didn't last long, by 1825 John Q Adams and Andrew Jackson were battling it out for leadership, and Americans' view of the United States as singular suffered accordingly.
Nevertheless from barely 10% in 1840 "is" rose to 20% in 1860, 50% in 1880, and 80% by 1900.
Iirc, the end of "are" came in 1906 with an official act of Congress declaring "is" correct.
We should note that Secretary of State Richard Olney's 1895 defense of "are" -- Olney was a Democrat serving Democrat President Cleveland whose administration was heavily dependent on the Solid South Democrat votes.
Olney himself was from Massachusetts, Cleveland from New York, but these Northeastern Power Brokers' political & grammatical alliance with Solid South Democrats should come as no surprise to DiogenesLamp, who is very concerned about such things... ;-)
Democrat Richard Olney, Secretary of State under Democrat President Grover Cleveland:
