Two thousand years ago, the Roman poet Ovid encouraged young women seeking illicit lovers to write their suitors secret messages in milk, rendering their words of enticement invisible until touched with coal dust.

That sexy allurement is the way Georgia Institute of Technology professor Kristie Macrakis opens her intriguing book, Prisoners, Lovers & Spies: The Story of Invisible Ink from Herodotus to al-Qaeda.

“Secret communication is part of the game of love, and forbidden love heightens romance,” writes Macrakis, a Harvard-educated historian of science, who grew up reading true crime books, not romance novels.

But Macrakis, who is also an expert on espionage, confesses that if the story of former National Security Agency contractor Edward Snowden had broken while she was writing Prisoners, Lovers & Spies, she would have opened her book with the computer geek intercepting secret communication rather than the randy poet instructing women on the art of seduction.

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Certainly, Prisoners, Lovers & Spies captivates with its tales of early secret communication—a war message tattooed on a slave’s shaved scalp but sent only after the slave’s hair had grown back, a Jesuit priest’s escape from the Tower of London thanks to secret messages sent via orange juice and orange peels, and America’s (perhaps first) Congressional budget battle due to George Washington’s love of and use of invisible ink during the American Revolution.

Then there’s Carl Frederick Muller, a German living in World War I London. Like the Jesuit priest who escaped the Tower of London, Muller used citrus juice as an invisible ink. But Muller did it to alert the Germans to British troop movements. And the Brits, believing they had the right and power to read everyone’s mail, and believing that women—especially women with well-turned ankles—had a sixth sense to discover secret writing in mail, hired thousands of women to do just that. That interception of mail eventually led New Scotland Yard to Muller’s boarding house where they discovered three pieces of lemon and two pens in his dresser drawer and another lemon in his overcoat pocket. “By the time Carl Frederick Muller faced the firing squad at the Tower of London, he was calm,” Macrakis writes.

What the British did 100 years ago—blanket surveillance in order to catch spies—is exactly what the NSA is doing today. “It’s just much more high tech and all-encompassing,” she says. But 100 years ago, the United States believed that “gentleman don’t read each other’s mail.” So during times of war, surveillance and mail interception expanded, while in times of peace it contracted.

“And then the Cold War was so long that the war never really ended and it just set everything into place and it got bigger and bigger so that we don’t contract anymore. They just violate these civil liberties that people were used to having before,” Macrakis says. “Frankly, if people had been monitoring history, maybe we wouldn’t be where we are—there would be measures in place so that this wouldn’t have happened.”

For the Ph.D. in the history of science, all of her books—and this is her fourth—have grown out of personal experience. While studying at Harvard, the Boston native was awarded a fellowship that sent her to Communist East Germany. At the time, Macrakis knew nothing about spying, she says, except what she’d read in John le Carré novels. But after the fall of the Berlin Wall, she heard about the German Democratic Republic’s Ministry for State Security (Stasi) and secret police. She started investigating the Stasi, which created her interest in the history of espionage and eventually led to her first book, Surviving the Swastika: Scientific Research in Nazi Germany, and third book, Seduced by Secrets: Inside the Stasi’s Spy-Tech World.Macrakis_cover

Two years before Seduced by Secrets was published, Macrakis was once again in Germany submitting, as she puts it, “request after request to the Stasi archive for material relating to methods of secret writing.” Despite carefully perusing “miles and miles of Stasi files,” no secret writing formula ever appeared. Then one day, mixed among a pile of useless paper that she’d been handed, Macrakis found a thin file marked top secret, and in it she discovered the German’s World War I formula for invisible ink, a formula that America’s CIA still refused to declassify.

That formula developed into Prisoners, Lovers & Spies. And as she wrote the book, in 2011, the CIA finally released its World War I secret ink formulas.

Now Macrakis travels the world teaching and lecturing about invisible ink and its uses in both love and espionage. Because, in this day in age, you can be sitting in a Starbucks sending a love text to your illicit paramour, while the CIA, NSA, Edward Snowden and your spouse intercept it and release it to the media…or Kristie Macrakis. And no coal dust is needed.

Suzy Spencer is the author of the New York Times bestselling true crime book Wasted.