Julia Child, a spy? Though hardly a revelation (Child herself wrote about her secret-agent career), it’s one of those pairings that boggle the mind, like the fact that Richard Nixon composed for the piano or that Marilyn Monroe married Arthur Miller. A gangly six-feet-two-inches tall, the cheerful and utterly sensible television chef hardly seemed a seductive Mata Hari or killer from La Femme Nikita.

In fact, she was neither. She wasn’t really a spy at all, but rather a kind of office manager, albeit an effective one, for the Office of Strategic Services, an intelligence and covert-action agency created during World War II.

That’s precisely the problem for Jennet Conant — and for readers of her new book, “A Covert Affair.” The subtitle promises the story of “Julia Child and Paul Child in the OSS,” but derring-do the two did not. Paul and Julia, then Julia McWilliams (their romance and wedding came after the war), faced hardships, bureaucratic rivalries and the unfamiliar cultures of India, Ceylon and China, but they blew up no bridges and stole no secrets. Paul, for one thing, was an artist who designed war rooms.

That’s probably why Conant, whose previous books include “The Irregulars” and “Tuxedo Park,” devotes most of her pages to Jane Foster, an OSS colleague. Magnetic, mischievous and piercingly intelligent, Foster dominates the narrative. She was the life of her own moveable party, a quipping, knowing, dazzlingly dressed woman of the world. Born into San Francisco’s high society, she studied art in Europe, lived in Java, and traveled around the Pacific rim before joining the OSS. There she worked in Morale Operations, dreaming up loony schemes to depress Japanese soldiers.

Conant writes in a breezy, amiable voice, reflecting Foster’s wisecracking and tippling style. (If her career were a film, it would be a cross between “The Thin Man” and Bob Hope and Bing Crosby’s “Road” movies.) The light tone is winning, but there’s little narrative drive. One set of minor hijinks follows another, to no particular end. Ultimately I found the rollicking Foster to be less interesting than the more conventional Julia McWilliams. Conant nicely captures Julia’s transformation into a cosmopolitan, intrepid, yet still cheerful soul. The account of how her friendship with the older Paul matured into romance is particularly good. He was quite full of himself, initially dismissing Julia as both “a grown-up little girl” and “an old maid,” but becomes an increasingly appealing character as he gradually realizes that Julia loves him.

Foster, on the other hand, remained a seemingly frivolous person who never grew — with one important exception. At war’s end, she was sent to the Dutch East Indies (now Indonesia), where she protested against U.S. support for the Netherlands’s brutal suppression of the independence movement. She alienated the State Department, which inherited the remains of the OSS. When McCarthyism marched in, she was investigated, hounded and ultimately indicted for spying.

Conant makes Foster’s treatment seem terribly unfair, but mentions legitimate reasons why Foster was a suspect. She had joined the Communist Party; her husband was a Russian emigre and outspoken radical; she was friends with Soviet spies and gave her classified OSS report on Indonesia to one of them.

Then, at the end of Foster’s saga, Conant writes, “Jane admitted to passing on some material” to spies, and won asylum in France by giving information on Soviet espionage there. This late revelation deflates the drama of her purported persecution. I wondered if the real story was that her frustration with U.S. policy drove her to dabble in actual spying, making her a more substantive (and dangerous) person than the one Conant portrays.

Conant does write vividly — maybe too vividly. Here’s her description of Foster’s reaction to her colleague Betty MacDonald’s story of being recruited: “Jane snorted, then shot a cautious glance at the fingerprint matron to make sure her outburst had gone unnoticed. She refrained from commenting on Betty’s account . . . but her eyes were bright with amusement. She arched her eyebrows theatrically, silently acknowledging what they both already knew: this mysterious crew had to be a secret service organization of one stripe or another.”

Wow. I rarely find such acute observations when I conduct research. But the end notes appear to credit only MacDonald’s memoir, which merely says: “I explained all this to Jane. It turned out that she had been hired under similar circumstances.” Of course, Conant may have drawn on additional, uncited material. But such fine detail in nonfiction invites incredulity; the sources should be clearly identified and treated with healthy skepticism by the author.

Perhaps most readers won’t — and shouldn’t — care about such scruples. This is not the place to look for searching analysis or scholarly precision. Though Conant makes a few confidence-sapping blunders (she describes the Japanese as chasing the French out of Indonesia, rather than France’s actual colony of Indochina), she has written a yarn as much as a history. Whatever her book’s flaws, you can tell she had fun writing it, and the fun is contagious.

T.J. Stiles is the author of “The First Tycoon: The Epic Life of Cornelius Vanderbilt,” winner of the Pulitzer Prize and National Book Award. He is working on a biography of George Armstrong Custer.

A COVERT AFFAIR

Julia Child and Paul Child in the OSS

By Jennet Conant

Simon & Schuster. 395 pp. $28