It’s daunting to report a book about spies. The job of an intelligence officer, after all, is keeping secrets. As I embarked on the research for The Sisterhood, my seven-decade history of women at the Central Intelligence Agency, I wondered if even half a dozen women would come forward to share their stories.

I began posting discreet inquiries on e-mail chains and newsletters known in the intelligence community, the journalistic equivalent of placing a classified ad and crossing your fingers. To my delight, scores of women spies, both current and former, came forward. These were women who witnessed the Soviet quashing of the Hungarian Revolution in the 1950s, evacuated C.I.A. stations in Libya and Cambodia in the 1960s and 1970s, and served in Maiduguri, Tokyo, Moscow, and Geneva. Many had conducted crucial intelligence work despite the overt and unapologetic sexism that permeated the agency for decades. These women had a lot to say.

That’s not to imply that they told me things they shouldn’t have. The women knew what they could and could not reveal. But spies’ careers are so fascinating, their lives so surreal, their frustrations so monumental, that these women were eager to share.

What astonished me most is how young spies are. When you think about a spy, perhaps you envision Mark Rylance’s character in Steven Spielberg’s Cold War drama, Bridge of Spies, a man who is graying, sagging, burdened with regret, and disillusioned. Not so. Spies are babies. Many of the Cold War–era spies I spoke to for The Sisterhood had been recruited in their 20s, straight out of college. Today’s spies are millennials or even Gen Z–ers. Spies are like the Romantic poets or mathematicians, said to do their best work in their 20s.

National-security secrets are being collected by people still using their parents’ Netflix account. They know how to fire a Glock and how to elicit state secrets, how to case a meeting place, make a getaway, and scour a database for coordinates to set up a drone strike. If you are the parent of a young adult who recently graduated from college, just imagine that this kid, who maybe still needs you to co-sign a lease to rent a studio apartment, is the person responsible for meeting with heads of state in Asia, Africa, and the Middle East.

National-security secrets are being collected by people still using their parents’ Netflix account.

“I was like a teenage bride,” says Molly Chambers, a former C.I.A. case officer I interviewed, who received a recruitment call during her junior year of college. She thought it was a friend playing a prank. When she took a semester off her senior year, her sorority sisters thought she’d left to secretly have a baby. In fact, she had left to go to McLean, Virginia, to start her spy training. For her first posting, in Uganda, she showed up for work wearing Old Navy, all she could afford on a government paycheck.

The reason spying is a young person’s game is because it’s an excruciating, 24-7 grind. You work a cover job during the day, maybe processing visas as a consular officer in Moscow, sitting at the same window every day, getting spat on through the holes in the plastic screen. Then, at night and on weekends, you hold clandestine encounters in cars and hotel rooms, take long train rides, spend hours on a surveillance-detection route to get to meetings. You wear sunglasses and are expected to drink. The work is urgent, exhausting, and lonely.

Mike Kalogeropoulos, a retired male case officer I interviewed, recalled that when he took his first posting, in Athens in the early 1970s, the Greeks called him “the boy.” Another C.I.A. officer, a woman who is still undercover, recalled that on her first posting, in Pakistan, she would read People magazine so she could gossip about Michael Jackson with kings and Cabinet ministers. Her sources would giggle when she used American slang.

They took her seriously, though, and they thought she was cool. Among themselves, she and other female spies referred to their jobs—persuading foreign nationals to become “assets” and share secrets their own governments were trying to hide—as “operational dating.” By this, the women meant that when recruiting and handling an asset, they remained on their best behavior, willing to watch every season of Lost, share a computer thumb drive full of every single Keanu Reeves movie, whatever it took to win the trust of potential assets.

“We are like the perfect girlfriend,” she said. “We never let the crazy out.”

Liza Mundy’s The Sisterhood: The Secret History of Women at the CIA is out now from Crown