About 250 years ago, writers and journalists across Europe began to notice an astonishing — and, many thought, disturbing — change in human behavior. To some observers it looked like a “fever” or an “epidemic” or even a “madness.”Pessimists warned that terrible moral corruption would surely follow in its wake. Although if it was an epidemic, it was not one that a lot of people nowadays would find especially worrying. What was happening was this: In the middle of the 18th century, it suddenly seemed that everyone, everywhere had started to read.These new readers, one onlooker thought, seemed as “addicted”...