Eudora Welty, the Mississippi-born grand dame of Southern fiction, met William Maxwell, writer and New Yorker editor, at a party in New York in 1942. Maxwell, who had fallen in love with Welty’s work, longed to see her published in the New Yorker.

A few months later, Maxwell wrote, asking her to send him one of her stories. Welty replied, suggesting he contact her literary agent.

By the time “The Bride of the Innisfallen” finally ran in the magazine in 1951, their correspondence had launched a lifelong friendship during which they would exchange hundreds of letters, more than 300 of them collected in “What There Is to Say We Have Said,” edited by Welty scholar, longtime friend and biographer, Suzanne Marrs.

Welty, one of our most beloved and celebrated writers, needs no introduction; but Maxwell, whose novels and stories have only come back into fashion in the past decade or so, hasn’t enjoyed the same uninterrupted popularity.

At the New Yorker from 1936 to 1976, he worked with a stable of writers that included John Updike, J.D. Salinger, John Hersey, Vladimir Nabokov and John Cheever. Born in Lincoln, Ill., in 1908, Maxwell wrote short stories and novels, as well as an award-winning memoir (“So Long, See You Tomorrow”). He was known to possess a genius for intimacy, a reputation for setting others at ease. The same was said of Welty.

No wonder these two were wild about each other, trading likes and dislikes with lightning speed, discovering “tandem” backgrounds that drew them all the closer: They were born within a year of each other, in small, sleepy towns where “everyone knew the name of everybody else’s horse and dog.” Both lost a parent early on, attended a Midwest college and moved to New York at the earliest opportunity.

Marrs, a professor of English at Millsaps College whose 2005 biography of Welty met with mixed reviews for its focus on the minutiae of the writer’s life, here again presents a raft of tender, day-to-day details, including the growing of roses and the exchange of cuttings, the making of kumquat jelly and watermelon rind preserves.

Like eavesdroppers on a party line, we’re privy to everything from Welty’s running account of her mother’s cataract condition to daily weather reports, recent home repairs, and the endearing habits of Maxwell’s newborn daughter: “[Kate] loves people with low voices, and would prefer that strangers speak in whispers. Perhaps we all would.”

But for every letter where “doves sing, a lizard came out in the wall at noon, & the tea roses are still in green leaf,” there’s one showcasing two avidly curious, penetrating minds at work, weighing and analyzing the novels, essays and letters of the writers of their day. They wrote of their friends, other literary luminaries — Isak Dinesen, Elizabeth Bowen and Reynolds Price, to name a few — and swapped extensive reviews of art openings, films, opera performances and their frequent travels in Europe.

As Welty’s editor, Maxwell sent proofs of her stories, along with requests for changes or deletes. The resulting exchanges provide irrefutable evidence of the keen attention to detail that made Maxwell one of the most respected editors in his field, and reveal the care he took to maintain the integrity of Welty’s prose style: “‘To the good of the heathen’ would be a legitimate idiom,” she assures him in one reply; in another, confirming “there is a little early June peach, though mostly they ripen in July.”

They were shameless advocates for each other’s work. Of Maxwell’s story, “What Every Boy Should Know,” Welty wrote, “It works its powers on those places in the heart and mind that most stories never reach, and don’t know how.” Maxwell tells her “Losing Battles” is “a comic masterpiece” that reminds him of “Tom Jones” and “The Tempest.” Upon finishing “Golden Apples,” he gushes, “This is how one feels in the presence of a work of art.”

When difficult subjects came up, they retreated politely into their shells, like two sensitive crabs: Throughout Welty’s decade-long romantic involvement with married mystery writer Ross MacDonald (aka Kenneth Millar), a discreet silence on both sides prevailed. Similarly, when Maxwell quit the New Yorker to pursue his own writing, rather than express her dismay, Welty simply stopped writing him for a few months.

Yet their mutual devotion was unshakable, and continued until the very last letter, sent by Maxwell on Welty’s 90th birthday.

In today’s world of texting, Twitter and Facebook, where our empathy for others is often reduced to a “like” button, coming across such a sustained account of a friendship is like shining a flashlight on the cave walls at Lascaux: Is the warmth and passion and the profound desire for understanding that permeates these letters on the verge of dying out?

Surely we can learn something from Maxwell when, anticipating some rose cuttings Welty plans to send him by air, he writes so unashamedly, “How I long to have this connection, this kinship, between your garden and ours established.”

How fortunate we are that their kinship endured long enough for them to say everything there was to say.

Nonfiction

What There Is to Say We Have Said: The Correspondence of Eudora Welty and William Maxwell

Suzanne Marrs, editor

Houghton Mifflin Harcourt, $35, 512 pages