IN SOME WAYS, Sally Fitzgerald knew Flannery O'Connor better than anyone.

She knew O'Connor's message: That belief in Christ is a matter of life and death. Fitzgerald shared the belief.

She understood O'Connor's medium: A peculiar violence and a rare comic touch.

And more than anyone else recently, Sally Fitzgerald has been responsible for the swelling ranks of O'Connor readers. The source for this surge of interest is the book edited by Fitzgerald, The Habit of Being: Letters of Flannery O'Connor . Last month it was given an unprecedented special award by the National Book Critics Circle.

Letters -- "beautifully put together in every sense," according to one reviewer -- sheds more light than anything yet published on the 31 stories and two novels written by O'Connor before she died at age 39 in 1964 of the rare disease lupus erythematosus.

Yet the Fitzgerald-O'Connor friendship made the Letters neither an obvious nor a simple undertaking; to the contrary, O'Connor's remark about the difficulties of writing seems to apply similarly to Fitzgerald: "I work from such a basis of poverty that everything I do is a miracle to me."

Fitzgerald, 63, mother of six, grandmother of one (shortly to be three, she says), had written very little before -- some reviews many years ago for Commonweal and The New Republic. Moreover, complications in her own life delayed her work and sometimes caused her to wonder whether she would start it, much less finish it. Yet she wrote a graceful introduction and the judicious, informative connecting passages that bind the letters so exquisitely. Fortified by the book's reception -- 25,000 hardcovers sold after an initial printing of only 5,000 -- Fitzgerald is currently at work on a biography of O'Connor.

When she is not doing research in O'Connor country, Milledgeville, Georgia, Fitzgerald works out of a small office without a telephone on the Radcliffe campus in Cambridge, Massachusetts; she has a two-year grant to work on the biography from Radcliffe's Bunting Institute. We talked there at length twice about her work and the Letters . Among the few books on her shelf are Milledgeville: Georgia's Antebellum Capital, Flannery O'Connor and Caroline Gordon: A Reference Guide and Simone Weil's Waiting For God. A slender woman with gray hair, Fitzgerald prefers to talk about O'Connor rather than herself: she'll often laugh when recalling a particular O'Connor witticism, but she'll sometimes seem uneasy and nervously work a tissue in her hands when the conversation turns to her own life.

Fitzgerald exudes refinement, a certain elegance. She's careful to speak grammatically. But her rather formal demeanor seems offset by a vivacious underside -- the easy laugh, a rare energy and determination at an age when many consider unrestrained leisure the ultimate achievement.

Fitzgerald and O'Connor, enthusiastically brought the young writer to the New York apartment of Fitzgerald and her husband Robert, the poet and translator. "The most striking thing about her," Sally Fitzgerald said, "was her shining gaze, her quite beautiful eyes. She didn't say much but she seemed to take in a great deal." Observing and listening, Fitzgerald would learn, were O'Connor's great gifts.

That summer, looking for someplace to finish her first novel, Wise Blood, and thinking (wrongly, it turned out) that she had to get out of the South to accomplish her work, O'Connor moved into the Fitzgeralds' new home in Ridgefield, Connecticut. It was a large fieldstone house surrounded by oaks on the highest land from there to New York City, perfect solitude for O'Connor with some minor exceptions, such as the growing Fitzgerald brood.

Still, there were enough quiet times in the year and a half she spent there for 'o'Connor to think and write in her room above the garage; over the dinner table to gain insights into writing and reading from Robert Fitzgerald; to forge her friendship with Sally Fitzgerald. The women shared both casual and passionate beliefs. Both Southern emigres in a sense (Fitzgerald considered herself that, having been raised in Texas before attending the University of Southern California and moving to New York City), O'Connor and Fitzgerald laughingly disparaged the notion of the Southern Belle. Both deeply Christian, they believed in the mysterious action of devine grace, even in the most unlikely places. That faith forms the thrust of O'Connor's writing and the basis on which Fitzgerald could offer an occasional suggestion as the two washed dishes on a quiet Saturday night.

O'Connor's Connecticut outings principally were to mass, early every morning. "She wasn't interested in going off to New York," Fitzgerald said. "She didn't want to 'get away.' If she was snowed in or iced in, that didn't bother her. She would write in the morning, come down to lunch and nap or baby-sit in the afternoon. There was no company but our own. We were just very happy."

O'Connor was not much interested in self-adorment and, according to Fitzgerald, her attire on their first meeting proved typical: "A bunchy coat, slacks and sweater, and a plaid shirt."

"There was a kind of frailty about her, a softness," Fitzgerald said. "Obviously she had not been athletically inclined. Her voice was slow and southern, remarkable even in Georgia for its pace.

"But when I read 'The Train,' I knew I was in the presence of a formidable talent."

In December, 1950, O'Connor complained of feeling ill and returned to Georgia, where she was hospitalized, almost dying from the lupus that would eventually kill her. She would live most of the rest of her life on a farm outside Milledgeville called Andalusia, where she was nursed devotedly by her widowed mother Regina, now 84, while writing and maintaining a correspondence with anyone who could keep up with her.

She visited the Fitzgeralds three times more, briefly in Connecticut in the summers of 1952 and 1953, and in Italy during 1958, where the family had moved while Robert Fitzgerald translated Homer. O'Connor tied that trip in with a visit to Lourdes -- the hope of a cure being not hers but her octagenarian cousin Katie's.

"It is Cousin Katie's end-all and be-all that I get to Lourdes," she wrote to the Fitzgeralds, "and if I am dead upon arrival that's too bad but I still have to get there."

"Flannery hated to turn her down," Fitzgerald said. "But, actually, she rather dreaded the idea of a cure. She could handle the illness. But she didn't want the vocation of a miraculous recovery.

"Her cousin Katie died that November terribly happy that Flannery had done this."

Living in Italy, Fitzgerald never saw O'Connor again after 1958, but their correspondence is an important part of the book, as are letters to Robert Lowell and Elizabeth Hardiwck, Elizabeth Bishop, John Hawkes, Walker Percy, Caroline Gordon and others. But the letters that form "very much the backbone of the book," according to Fitzgerald, are those to a friend who wished to remain anonymous and who is identified only as "A." In these letters, O'Connor's spiritual strength comes through, described by Fitzgerald as "quite simply a Crist-centered life -- not that that's all you think about, but that around which everything revolves."

"One of the awful things about writing when you are a Christian," O'Connor wrote to "A," "is that for you the ultimate reality is the Incarnation, the present reality is the Incarnation, and nobody believes in the Incarnation; that is, nobody in your audience."

To get people's attention, and make her point, O'Connor employed violence: "The novelist with Christian concerns will find in modern life distortions which are repugnant to him, and his problem will be to make these appear as distortions to an audience which is used to seeing them as natural; and he may well be forced to take even more violent means to get his vision across to this hostile audience." It worked for her, if not every reader: "Violence is strangely capable of returning my characters to reality and preparing them to accept their moment of grace."

It seems to Fitzgerald, while putting together the Letters, that those mysterious interventions of grace that O'Connor talked about might be occurring even during her assembling of the book. "There was no clap or revelation and Flannery was standing by my side," she said. "It was just a realization that she was still my friend and still helping me out when I needed it."

There was the time, for instance, that Fitzgerald recalled O'Connor having mentioned the name of that crucial correspondent who wanted to be anonymous, and again when "A" finally allowed her to have the letters published. "It finally allowed her to "It was very much a cliffhanger," Fitzgerald said. "First she said she would, then she wouldn't, then she would."

The offer to do the Letters for the publisher Robert Giroux -- who, along with O'Connor, had been a godparent of the Fitzgeralds' third child -- came in the midst of her anguish over the breakup of her marriage of 25 years. She does not talk easily about this, in fact only when pressed.

"I felt as though my arms and legs had been cut off," she said. "My emotional state was comparable to one's physical state after a serious auto accident.

"It seemed to me almost as severe as Flannery's encounter with her own destiny, the illness that would circumscribe her own life."

She said that for a year after the offer came in 1975 she "couldn't cope with the letters." and that her publisher was extremely patient. (With her husband, she had previously edited for Giroux Mystery and Manners, a collection of O'Connor essays, lectures and articles.)

She put off Giroux and took first a part-time, eventually a full-time job as a secretary at the Harvard law school. And she prayed -- "I didn't make any production of it," she said, sounding much like O'Connor herself. Eventually, "it began to weigh on me that I wasn't meeting my commitment," and she began tracing letters. And solidifying the confidence of Regina O'Connor, no small accomplishment according to those who know her reluctance to have anyone involved with her daughter's possessions.

Most of all, Fitzgerald began to organize the letters and get her own words on paper.

All of this, she said, took to her own amazement.

She operated out of a briefcase, which almost proved disastrous. She relates an incident that occurred at her home in Cambridge, where she has lived the greater part of the last 16 years and now lives with two daughters, a son-in-law and grandson.

"I was carrying around two years of original letters," she said. "I came home one Saturday from working on the book at the law school. It was about 7 in the evening. I put the briefcase down in the entrance hall.

"The next morning I woke up about 5 o'clock. I made myself some tea, the house was quiet and I started looking for the briefcase. I ransacked the house and thought I must have left it at the law school. At 5:45 on a Sunday morning I met the Harvard police to let me in the office. It wasn't there.

"When I got home I noticed the window was open. Someone must have been inside when I came downstairs and he got out the window, grabbing the first thing he could find. Of course it contained the greatest treasure of my life.

"I saw footprint and looked around the garden and he had gone behind a big rhododendrom and opened the briefcase and scattered the letters over the face of the dewy earth."

The wind was not blowing that morning.

"It was a miracle I got them all back," she said. She went immediately to the pastor of St. Paul's Church in Cambridge, refuge of poets and paupers, and received from him a tiny room in the rectory where she finished her work.

When the 617-page Letters hit the bookstores in Harvard Square and immediately sold out, one native said, "No one could quite believe she had done this," as if it were beyond the capabilities of a housewife and mother.

Fitzgerald laughs. She seems more interested in what people think of O'Connor, and if Letters is a triumph, she attributes "most of it" to her connection with O'Connor, a feeling that she still has "recourse" to her."And I hope," she adds, "I have it for this biography, which is much more difficult than anything I've ever undertaken."

The Habit of Being: Letters of Flannery O'Connor will be issued this month as a Vintage paperback. Fitzgerald also mentions this month's U.S. opening of the film Wise Blood, based on the O'Connor novel. This film has been playing in England and France. Directed by John Huston, it was coproduced by Fitzgerald's son Michael and his wife Kathy, and written by another son, Benedict. It also benefits from Sally Fitzgerald's artistic touch. Shortly after college, she had aspired to be an artist and studied in Ney York. Asked by her children to join in the film's production, she happily went to work designing costumes and sets. CAPTION: Picture 1, Sally Fitzgerald, By Laura Levine; Picture 2, Flannery O'Connor; Copyright (c) By De Casseres