Among the love stories in American literature, one that is most enduring did not unfold in a novel but in real life.

In 1918, Lt. F. Scott Fitzgerald met Zelda Sayre, the 18-year-old daughter of a judge, at a country club dance in Montgomery, Ala., where he was stationed. That chance encounter led to a 22-year romance that would become the subject of biographies and movies, accounts told from an outsider’s perspective. But in “Dear Scott, Dearest Zelda,” a collection of more than 300 letters they exchanged, the reader hears their own distinctive voices. Zelda writes in direct yet passionate prose, Fitzgerald with a poetic flair reminiscent of his fiction. The result is an engrossing account of their love story — full of longing and ardor, heartbreak and betrayal.

On first meeting, Fitzgerald was smitten with Zelda’s charm and beauty, but the letters she wrote him deepened his affection. Discharged from the Army in 1918, Fitzgerald settled in Manhattan to realize his dream of becoming a writer. Zelda wrote from Alabama: “You know I am all yours and love you with all my heart.” And: “[O]ur fairy tale is almost ended, and we’re going to be married and live happily ever afterwards.” Her letters reveal that if there was any trepidation about their union it did not come from her but her mother, who feared Fitzgerald could not support her. “Momma gave me this today,” Zelda wrote in May 1919 — yet another newspaper clipping about a failed writer.

But Fitzgerald’s 1920 novel, “This Side of Paradise,” was an immediate bestseller. A week later, Scott and Zelda were married. Over the coming years, they exchanged few letters because they were almost always together. In an often turbulent lifestyle that included constant drinking, occasional infidelity and intermittent quarreling, Fitzgerald published a second bestseller, “The Beautiful and Damned”; they had a daughter, Scottie; and, living on the French Riviera, they became part of the smart set surrounding Sara and Gerald Murphy that included Pablo Picasso, Dorothy Parker and Ernest Hemingway.

When his third novel, “The Great Gatsby,” met with mixed reviews and poor sales, Fitzgerald descended deeper into drinking and Zelda lost herself in an obsession with becoming a ballerina that culminated in a nervous breakdown, causing her to be hospitalized in Switzerland from June 1930 until August 1931. The letters they exchanged were poignant. “We ruined ourselves,” Fitzgerald wrote. “I have never honestly thought that we ruined each other.” Zelda countered: “Please don’t write to me about blame” — and soon told him he should “start whatever you start for a divorce immediately.” But Scott’s very next letter declared: “I love you with all my heart because you are my own girl and that is all I know.” Zelda acquiesced. “I am better. It’s ghastly losing your mind.”

Upon her release, Zelda spent the next five years occasionally living with Scott and Scottie but mostly hospitalized in sanitariums in the United States. Their letter-writing was as much a lifeline for Scott as it was for Zelda. When his fourth novel, “Tender Is the Night,” failed, Fitzgerald found solace in their relationship. “[W]e haven’t been happy just once,” he wrote, “we’ve been happy a thousand times. . . . Forget the past . . . and turn about and swim back home to me.” That she could not was heartbreaking to them both. “[S]ince we first met,” Zelda wrote, “I have loved you with whatever I had to love you with.” Sadly, other — uncontrollable — forces intervened.

By 1937, Fitzgerald was out of money. To finance their way of life — clothes and travel, Zelda’s hospitalization, Scottie’s private school and then Vassar College — he returned to Hollywood. He began dating Sheilah Graham, a gossip columnist, but his love for Zelda remained. “Oh, Zelda,” he wrote. “Once we were one person and always it will be a little that way.” Zelda reflected: “And I love, always your fine writing talent, your tolerance and generosity. . . . Nothing could have saved our life.”

The last time they were together, on an ill-fated, alcohol-fueled trip to Cuba in April 1939, ended with Scott hospitalized in Manhattan and Zelda having to find her way back to her sanitarium in Asheville, N.C. Returning to Hollywood, Fitzgerald barely scraped by as studios balked at hiring him because of his drinking.

A nadir came in 1940. “[M]y God,” Fitzgerald wrote to Zelda, “I am a forgotten man. ‘Gatsby’ had to be taken out of the Modern Library because it didn’t sell.” Still, he forged ahead on a new novel, “The Last Tycoon.” “I am deep in the novel,” he wrote to Zelda, who, having been discharged, lived in her family home in Montgomery.

In a letter in early December, Fitzgerald mentioned a heart problem. “This is not a major attack but seems to have come on gradually.” Another letter: “The cardiogram shows that my heart is repairing itself.” But, on Dec. 21, 1940, as he visited with Graham, Fitzgerald collapsed from a heart attack. He died before an ambulance could arrive.

Over the years, myths have emerged around their romance — Zelda drove Scott to drink, Scott pushed Zelda into madness — but their letters portray something else: a singular, enigmatic connection. “Scott and Zelda stayed in love until the day they died,” Eleanor Lanahan, their granddaughter, writes in a moving introduction to their exceptional letters. “Perhaps it became an impossible and impractical love . . . but it was a bond that united them forever.”

Paul Alexander is the author of seven books, among them “Rough Magic” and “Salinger.” He teaches at Fordham University and Hunter College in New York City.

Dear Scott, Dearest Zelda

Scribner. 432 pp. $22