If you sent a letter in 17th-century Europe, there was a good chance it would pass through one of the continent’s so-called “Black Chambers”—secret rooms attached to post offices and staffed by intelligence units, where mail was opened, copied, resealed, and sent on its way, with the writer and recipient none the wiser. Nadine Akkerman, a senior lecturer at Leiden University, is an expert in 16th- and 17th-century espionage. But during her research into the Black Chambers, she ran across something perplexing—a document by Samuel Morland, a British spymaster, in…
