“As Always, Julia: The Letters of Julia Child and Avis DeVoto,” by Joan Reardon; Houghton-Mifflin; 416 pages; $26.

In 1952, a middle-aged American woman living in Paris responded to a complaint about the poor quality of American-made kitchen knives by one of her favorite magazine writers and sent him a couple of French-made ones from her neighborhood store. If you believe history turns on specific moments, you could say that impulsive act of generosity was the start of the American culinary revolution.

The housewife was Julia Child, and if her “Mastering the Art of French Cooking” and accompanying television shows weren’t the catalyst for the change in American attitudes toward cooking during the 1960s, they were certainly its most recognizable symbols.

The writer was Bernard DeVoto, and though he is largely forgotten today, in the 1950s he was one of the country’s leading public intellectuals, a historian, author and longtime columnist for Harper’s magazine, when that really meant something.

DeVoto didn’t answer the letter himself, of course; that was left to his wife, Avis. And for that we can be thankful because her gracious thank-you letter to Child led to a long correspondence, which has been captured in Joan Reardon’s new book “As Always, Julia.”

If Child was the mother of the modern American interest in cooking, Avis DeVoto was its midwife. She’s the one with the publishing background who steered the novice author through the perilous shoals of the book world, connecting her with publishers and editors, offering astute criticism, even copy-editing text, testing recipes and sending American ingredients to Child overseas so she could test with the same materials her audience would be using.

In the process, what began as a polite correspondence between well-bred ladies of a certain class blossomed into a kind of frank intimacy in which Child reveals sides of herself that were known to only a few of her closest friends.

While it can pretty safely be said that there are few people in America today who haven’t heard of Child, it is also true that there are very few who actually have an idea of who she was — as apart from her cookbooks and shows.

Until last year’s movie “Julie & Julia,” in fact, it’s possible that the iconic image of Child was Dan Aykroyd’s impersonation on “Saturday Night Live,” which hilariously distilled the essence of her loopy, semi-hysterical TV personality. Nora Ephron’s marvelous movie revealed a bit more of the person behind the caricature, but ultimately, it was just a tease compared with what’s found in Reardon’s book.

The real Child was infinitely more complex and much more interesting.

Just as interesting is the far subtler picture that is painted of these two women, the times in which they were living and the friendship that grew between them.

And make no mistake, Child’s book didn’t come easily. It took more than 10 years from the time she started work on it until publication in 1961 (when she was almost 50). And along the way it passed through two other publishers before finally finding a home.

Through everything that transpired in those 10 years — successes and failures, news and gossip, births and deaths — their friendship deepened.

Though “Mastering the Art of French Cooking” was definitely Child’s own work, along with co-authors Simone Beck and Louisette Bertholle, there’s no mistaking how much she came to rely on Avis DeVoto’s advice. It was DeVoto who guided Child from the small publisher she had started with to the much more commercial Houghton-Mifflin, and then years later after they rejected the book, she hand-carried it to friends at Alfred A. Knopf, where it was eventually published.