No country is more deeply anchored by the free press than the United States, whose founders made the media the only industry specifically protected in the Constitution. The press was so important, Thomas Jefferson wrote in 1787, that “were it left to me to decide whether we should be a government without newspapers or newspapers without a government, I should not hesitate for a moment to prefer the latter.” And yet, as Jeff Jarvis notes in his fascinating new book “Hot Type: The Magnificent Machine That Gave Birth to Mass Media and Drove Mark Twain Mad,” very few Americans read the newspaper at the time of the nation’s founding.

The issue wasn’t primarily educational, Mr. Jarvis notes, but technological. Printing a newspaper required workers to painstakingly place individual letters on a metal plate and manually pull a crank to print individual copies of each and every page. Once printed, type had to be cleaned with lye and, reportedly, sometimes, urine.…