DARLINGHISSIMA; Letters to a Friend. By Janet Flanner. Edited and with commentary By Natalia Danesi Murray. Random House. 507 pp. $24.95.

FOR 50 YEARS, under the pseudonym Genet, Janet Flanner contributed installments of her "Letter from Paris" to The New Yorker. For more than 30 of those years, another series of letters -- more intimate and less concerned with appearances -- went to Natalia Danesi Murray, the woman she loved. "Darlinghissima" she called her, and this open record of what Mrs. Murray terms their "passionate friendship" shows why.

Murray was 38 when she met Flanner, who was 10 years older and already the striking "American in Paris" whose presence there would become better known than Benjamin Franklin's. Both women were once-married, both engaged in public careers -- Flanne as interpreter of the arts and politics of Europe for Americans; Murray as a broadcaster conveying to her Italian countrymen under fascism a sense of "the American way of life, its spirit and its freedom." Flanner had chosen Paris and expatriation over Indianapolis and the gentle austerities of her family's Quakerism. Murray had chosen New York and American citizenship over Mussolini's Rome and the state of being "Catholic, by tradition." Self-exiled by choice, cosmopolitan by nature, they adopted one another at once and for good.

In her first, V-mail, letter written in 1944, Janet chides herself for dealing with "tenuous material idiocies" rather than "items of deep consequence." In her final letter, written in 1975, she is troubled, equally, by the obscurities of Henry James' "book on the Initiates" and Lanvin's failure to provide "any new styles for tailored suits." First to last, her fleeting "material idiocies" give the letters a breeziness of address and unguarded feeling that nicely complements the more formal dispatches cabled from France to The New Yorker.

But even when she lets her (silver grey) hair down, Flanner's zeal to know, to understand and to communicate is apparent. Seldom have love letters contained so high a quotient of political discourse, or more confidently assumed that the beloved is as glad to hear the results of a debate in the Chamber of Deputies as to be told that she is "a woman in a million in mature elegance, charm and sensuality."

THE LETTERS are organized by decades and, along with Mrs. Murray's commentary, show social history in close-up. Vacationing on Capri in 1952, Janet goes to a party for Beatrice Lillie and there is introduced to an outgoing and attractive young congressman on crutches named Kennedy. Laughing, his hair windblown, he tells her he'd met with an accident in Sicily and presents the dark Sicilian girl who's traveling with him. Six years later -- alo with Katherine Anne Porter and Nadia Boulanger -- she receives an honorary degree from Smith College in a ceremony which includes "an impassioned speech for women's rights" by the same young man who is now Massachusetts' junior senator. On November 22, 1963, she writes: "About eight o'clock the phone rang at Josette Lazar's flat where I was dining with her, and Vladimir Vogey, a Petrograd artist -- the one who did that Cubist drawing in the Shawns' flat, which I gave them -- and heard Josette say, 'Kennedy shot. Oh, no. Not dead -- it can't be true.'"

Her list of acquaintances omits no one of any consequence anywhere; the number of those she counts as friends is almost as prodigious. In the security of her relationship with Natalia, she comments freely on figures from both and, with every opportunity to indulge in gossip for its own sake, never does. Her judgments of others are no more rigid than those she applies to herself. Falls from grace, however distressing, are all too human and explicable, and yet she can also call her own shots. "Take Tennessee. His imagination is a dire dramatic cesspool which fortunately spills onto the stage where it still shocks me, instead of into murders or destructions in bars or motels. But he is a real hater, seeing as an idealist (true? I wonder) only the hateful side of life and love, in a continued fury . . . of dissatisfaction."

She is as deeply fascinated by developments in music as she is by what transpires in Parisian painting. With an eye and ear uncorrupted by estheticism, she writes of these with a kind of perception charged most by the phenomenon of creation itself, yet sharp in its local discriminations. She had the down-home-in-Indiana horse sense to point out that "Inch by inch, a C,ezanne landscape is the highest priced newly discovered land known in the Western World." Along with Gertrude Stein, she dared to say of Scott Fitzerald: "My God, his superiority as an artist over Ernest Hemingway is incomparable. Ernest was a recorder; Scott a novelist." Even her dear friend Leonard Bernstein did not escape her finely-tuned ear and ready wit: "He must have had perhaps thirty or forty curtain calls. It was as extravagant as his programme, direction and also solo playing of concertos by Bach, Mozart, Ravel, then Gershwin's blues in which later he missed as many top notes . . . as if he had been Rubinstein himself."

A fervent believer in democracy as an actuality and as an ideal, she was nevertheless independent enough to put her faith in leaders whom party Democrats regarded as puppets of anti-democratic realpolitik. She saw Dwight Eisenhower as the genial representative of grass roots America, Charles de Gaulle -- however stiff-necked and imperious -- as the revenant soul of civilized France. When each of these men failed to meet the expectations her imagination had projected upon them, she owned up to disillusion with no loss of faith in her own standards.

In the annals of American character, Janet Flanner is both eponymous and unique -- the corn-bred woman of the world who meets that world on its terms and yet reserves for herself a degree of skepticism Europeans find congenial and a depth of idealism that baffles them. The balance is neither delicate nor achieved, but inherent. Flanner was a woman who could in the course of a morning write a brilliant encapsulation of the political turmoils of the continent and, that afternoon, cross Paris to make sure that lonely Alice Toklas -- surrounded by millions of dollars worth of paintings she was too proud to sell -- had enough coal for her grate, enough tea for her breakfast. She lived the life of her times to the full and was blessed with the talent to record it with a breadth of vision encompassing the permanent without scanting the quotidian. The substance of these letters is the values she lived by, all of them shared by Mrs. Murray herself, who concludes her introduction in the spirit they embody and exemplify.

"In rereading," she says, "I realized how unique our relationship was, worth sharing . . . not only for the . . . letters per se, but also as a demonstration of how two women surmounted obstacles, trying to lead their personal and professional lives with dignity and feeling. I hope that my grandchildren, and other young men and women like them, born in a freer, more liberated society . . . without the inhibitions or taboos of an earlier era, will understand and value our experience and efforts to be, above all, decent human beings."