You might remember where you were on July 19, 2007. In New York City, it was sunny and cool, with a scattering of popcorn clouds sweeping overhead. Most importantly, it was a Thursday—two days before the official release of the final Harry Potter book. And yet, on every newsstand that morning was a New York Times book review titled, “An Epic Showdown as Harry Potter Is Initiated Into Adulthood.”

By the fifth paragraph, readers were told, among other spoilers, that half a dozen beloved characters died, that a war had begun, that Voldemort’s followers breached the walls of Hogwarts and that Harry and Voldemort fought.

Anger flooded the internet. J.K. Rowling said she was “staggered that some American newspapers would publish purported spoilers in the form of reviews in complete...