By copyright expiry, public domain in the US begins in 1927. Later works may be in the public domain, but all works published prior to 1927 are in the public domain in the United States.
(This may not be the case in other countries.)
There were not many published books prior to the invention of the printing press, and many of those didn't survive. The total number of books (not individual titles but actualy bound volumes entirely) in Western Europe as of 1400 may have been as few as 50,000.
By 1800, about 1 million titles had been printed.
Over the course of the 18th century, presses became vastly faster, as they evolved from hand-operated wooden screw-press to iron-frames to steam and electric-powered rotary and ultimately web presses. Paper became much cheaper (and less durable --- a factor commented on at length in the Librarian of Congress's annual reports to Congress in the late 19th century). Literacy exploded from ~25% to 95%+ over the 19th century (and probably accounted for numerous revolutions and political upheavals).
Through much of the 20th century, certainly by 1950, US publishers were issuing about 300,000 new titles per year, a rate which state remarkably constant through the early 21st century. By the aughts, "nontraditional" self-publishing (a/k/a "vanity press") was nearing or exceeding 1 million titles per year more than had been published through all time to 1800.
Reports that all recorded data was doubling every few years date to at least the 1960s. That would mean that in any two year period ... half of all recorded information was less than two years old.
The catch is that not all recorded data is published. So I'm not sure what the time-distribution of all publishing looks like. But I'm pretty confident it's skewed far more recently than 1927. And would thus tend to be copyrighted rather than uncopyrighted.
If you want to measure works by significance, you might make a different argument --- there are many great works of literature, philosophy, history, and religion which were first published before 1927. But ranking and tabulating these is more challenging than a simple enumeration.