"Oh, c'mon now," an interviewer once hissed to novelist Belva Plain's publicity person, "Tell me what her real name is."

"Sure," said the publicist: "Fifi LaRue."

Belva Plain (whose real name is Belva Plain) loves to tell that (true) story.

"It's such a good name for a book jacket . . . it fits so well on the spine . . . everybody thinks it's a phony, and it's not."

Belva Plain's heroines are intelligent and independent women, even as she is, but she is quite content to use her husband's name as her own -- not just because it fits -- and she has no use for what she calls the "way-out radicals" of the women's movement who fault her for doing so.

In any case, she is the author of a pair of runaway bestsellers, first "Evergreen," in 1978, and now "Random Winds." The new novel is a saga of thwarted love, unspoken passions, illicit affairs, even some sensitively handled homesexuality. There is medical malpractice and a hint of the solid front the medical fraternity can present to the rest of the world, right or wrong. Of dedicated doctors and those who only say they are . . .

There is a little philosophy, a lot of human frailty, a bit of human strength and a healthy dollop of sex. "After all," says Belva Plain, "sex is an important part of life. First you have to eat. Then you have sex. Right?" i

Martin Farrell is the hero of "Random Winds." Son of a country doctor he is born into medicine, as it were, and the book clocks his professional and emotional maturation. It is, of course, as much about the three women whose lives he touches -- always, though never intentionally, with destructive result. How they -- the crippled first wife, Jessie, her artist-sister, Martin's true love, Mary Fern and Hazel, Martin's Illfated second wife -- deal with their love for the brilliant doctor and his inevitable betrayal of it, is the major thrust of the storyline.

And even as Martin manages to bungle his love life, so his professional competence and conscience mature until he sacrifices his crowning achievement rather than defend the actions of a drunken colleague.

"Random Winds" has been among the top five on the national list since April. And "evergreen" is still on the paperback best-seller lists, with 3 1/2 million copies in print. It's been translated into seven languages.

"I know," sighs the not-very-grand-motherly looking 60-year-old New Jersey grandmother, "now the next the line is something like 'not bad for a 60-year-old grandmother from New Jersey.'"

That is a line that Belva Plain is getting very tired of hearing.

"You would think," she says, from all the hoopla over the fact of her four grandchildren, that "I've just been writing grocery lists all my life and suddenly this semi-literate, dowdy little drudge from nowhere writes a besteller -- or two -- and isn't this all astonishing."

Actually, Belva Plain sold her first short story to Cosmopolitan when she was 25 and has been writing -- and publishing -- ever since.

Just not novels.

She is elegant and sophisticated, looking cool on a recent very hot day, and there is an ageless quality about her as she insists, "I don't mind being a grandmother at all." And adds "God willing, I'll be a grandmother of number five in October so I'm very happy about it." And then, with an unspoken "Let me tell you about my grandchildren," she tells this story about her grandchildren:

Her eldest grandchild is 7-year-old Matthew. He and his 6-year-old brother have taken to going into bookstores, and stationing themselves before the display of "Random Winds."

"THAT IS MY NANA'S BOOK," Matthew will bellow in his best glass-shattering tones. . . .

Belva Plain kvells.

Home for the novelist is South Orange, N.J., where she lives with her husband, Irving Plain, whose frail health recently caused him to give up his eye surgery practice. No, neither of her daughters nor her son is either a writer or a doctor. No, "Evergreen's" saga of a turn-of-the-century family of Jewish immigrants from Eastern Europe is not about her own family. No, the neurological-surgeon hero of "Random Winds" is not based on her husband.

"But of course," she says, "you take a little piece here and a little piece there, so a novel is a collage. I think that's the best way I can descirbe a novel of mine. A collage," she repeats.

"My grandparents came from Germany in 1875," Belva Plain explains. "I set 'Evergreen' in the 1900s for two reasons: It was more dramatic -- Germany was relatively quiet in the 1870s. And then, I wanted to bring the heroine up to the present, so 1875 would have been too long ago.

"So it's not my family, but it has relevance to my family and to the whole American experience, because as I find out from all the mail I have received from every possible ethnic group, it is always the same story of people coming to the promised land."

Belva Plain's succes fou has changed her life very little. She lives and works much the way she always has, writing for about half a day, making sure she gets out among people whose experiences can help her flesh out the characters of her imagination. She has a keen eye for detail, and what she observes will eventually be reflected in the pages of her books -- in, for example, the description in "Random Winds" of Mary Fern's English country house, from sculpured gardens to vermeil dinner service, of how it changed during the war . . ."He . . . watched her laying the places at the carved oak table, an earthenware plate and mug at each plate. He remembered her sitting at that table, wearing velvet . . ." or, of a New York street, ". . . He looked down to where, three floors below, bare gingko trees stood in a row along the curb, each one enclosed in its low fence of wire scrollwork."

She has a good ear, too, and is not above the occasional eavesdropping on indiscreet strangers.

About her own privacy she is adamantly protective. She will simply not discuss her financial affairs and is offended that anyone will ask. To the suggestion that her public is interested she will snap, "They're interested in prurience, too."

Belva Plain grew up in New York City. Her father was a builder. She went to the prestigious private, progressive Fieldston School run by the Ethical Society and then to Barnard.

She is cultured and sophisticated, well-traveled and soft-spoken. Her green eyes are perhaps a mite shadowed, but her high cheekbones, delicate features and flawless skin lend her a quality of youth.

She wears a loosefitting but a stylishly cut navy polka dot dress with a small gold cat pinned to her left shoulder. Her classic pumps and her crocheted bag are white. The overall effect is one of elegance.

Both Belva Plain and her books are ladylike. "Oh," she worries, when she is told this, "I certainly hope you don't mean prissy."

Neither she nor books could be called "prissy." Her characters are sensual, even earthy. But there is no obscenity, no explicit sexual descriptions.

"There is so much vulgarity in the world . . .," she says. Another small sigh.

Belva Plain's philosophy of her art is this: "Entertainment," she says, "is a very valid human need. If it is done with taste and some thoughtfulness and makes the person being entertained do some thinking too while he's being entertained, I think that's wonderful."

A Plain storyline is more saga than suspense. Her characters are people we could know; her protagonists may be noble of spirit but they do the same foolish things we all do. Their sucesses and failures appeal through the insights they give us into ourselves and people like them.

"I tried to capture the feeling you have for a doctor who really lives for his work," she says of Martin Farrell.

Plain novel number three is already moving to the front burner. She is coy about its plot, saying only that it is set in a foreign country and concerns politics.

Oh yes, politics. "Did you laugh at Reagan the other night?" she asks, "Or did you cry?"

She believes "Anderson's is a lost cause," and that our "process is so degrading, it is disgusting . . . so no decent people will run."

Of her personal political preferences she says, "You can say that like most thoughtful Americans today I'm appalled that we don't get a better choice than this. It's very, very sad.

"And you can say this, 'Oh don't I wish Harry Truman were back.'"