IT IS A SUMMER morning in a downtown cafe' in Monte Carlo, Shirley Conran's home town.

Across from the cafe', tourists gawk at luxury cars parked in front of the Hotel de Paris. The grand casino is to Conran's left, and to her right are the boutiques of the rich: Cartier, Yves Saint-Laurent and Bulgari. The wealth that is the backdrop for her best-selling novel, "Lace," is evident everywhere but she seems oblivious to the conspicuous consumption. Sipping Perrier, she hones quotes for her upcoming around-the-world book promotion tour.

Of her Swiss education following a childhood in war-torn London: "After years of deprivation, I chose snow and mountains and chocolate."

On writing about the jet set: "I've found that on the whole in life, kings are more interesting than bus drivers."

On marriage: "That way madness lies."

On her simple needs: "I like everywhere except Japan, and I like everything except celery."

And: "If you're going to write a novel, you might as well write an international one."

Conran's "Lace" hit the best-seller lists a couple of weeks ago, and so far everything is going according to plan. Forget the depressed book market or what experts say about the folly of launching a "major" (the publisher paid a bundle for it) book in the dead of summer. "Lace" was tailor-made to be a hit, from its conception to its cover--replete with satin sheets, lingerie, champagne glasses and caviar. Even the title's typeface was carefully chosen by the author.

"My specialty is writing things that you can't put down," says Conran. "I wrote it firstly as entertainment, but I think it's up to my standards, and I know I'm a good writer."

Set in glamorous capitals around the world, "Lace" details 30 years in the lives of four women who attended a Swiss boarding school together and grew up, each in her own way, to be rich and famous. Along the way, one gave birth to a daughter, Lili, and abandoned her. As an adult, Lili becomes a world-famous actress who wants to know which of the four women is her mother. The search for the truth is littered with opulence and decadence, earning "Lace," in the eyes of some reviewers, a place in the "schlock fiction" hall of fame, right next to the works of Jacqueline Susann and Judith Kranz.

Conran is ready for that kind of criticism: "When people know about the money," she sniffs, "nothing short of 'War and Peace' will satisfy them."

The money: If Tolstoy had been paid by the word for "War and Peace," he couldn't have dreamed of the kind of receipts "Lace" earned even before the hardcover edition was printed. According to Conran's New York agent, Morton Janklow, Simon & Schuster paid $750,000 for hardback and paperback rights, with escalators if the book reached the best-seller lists that could bring Conran's slice up to $1 million. British rights, including excerpt rights, sold for a whopping $250,000. At the Frankfurt Book Fair last fall, other foreign publishers read the first 17 pages shown them by Janklow and the book's editor, Michael Korda, and paid handsomely for reprint rights.

"Then I got back here and called the head of Lorimar Productions," recalls Janklow, "and within 24 hours made the biggest deal I'd ever made for a first novel for television. It'll be a six-hour mini-series for ABC by the executive producer of 'Dallas.' The amount is tied to escalators, but it'll be over $600,000 plus profit participation."

Eat your heart out, Tolstoy.

Conran knows something about money and the scramble for it. She was once married to Terence Conran, the British furniture magnate whose home furnishing chain, Conran's, is also a big success in America. The two met in the '50s outside London. She was an art student "marking time until marriage," he was opening a small cafe' that featured chairs and tables he'd designed.

Shirley Conran began working as a waitress in Terence Conran's cafe', and the two were married in 1955; she was 23, he was 24 and trying to get a break as a furniture designer.

"We were both very boy scout in those days," recalls Shirley Conran. "He owned one Savile Row suit that he needed, and I'd sew it up when he split the bottoms. I began doing Terence's public relations. We'd serve egg mayonnaise and spaghetti and lemon sorbet when we'd invite fancy people over for dinner--it was very continental but mainly very cheap."

Several years and two sons later, the Conrans began to prosper. She ran his fabric division and developed an interest in weaves. The Daily Mail in London asked her to become a design consultant and write one article a month.

She and Conran split up in 1962. He gave her an envelope with four weeks' pay inside. When she talks about it now, Shirley Conran sounds like one of the plucky heroines of her novel.

"I might say, in business terms, it was my fault--I had no contract, no stock . . . ." And Conran says it was two years before she and her estranged husband arranged a financial settlement that she emphatically says did not leave her a wealthy woman.

At the Daily Mail, "I just sat there because it was free and I hoped they didn't know I was using the phone to look for jobs at other papers," remembers Conran. She eventually became a full-time design writer at the Mail and was married for five years to the sales director of an automobile designing company.

In the late '60s, divorced again, Conran contracted viral pneumonia and stopped working.

"I began to have money problems," she recalls, though she says everyone always thought she was "tremendously" rich. "I had a five-story brownstone in London. I couldn't afford a maid and learned how to do housework." Soon she couldn't afford the town house, either, so she rented it out.

Finally, in 1975, she hit with a book that told in a breezy way how she coped with running a household with two sons and no help. "Superwoman" was "about housework and how to avoid it." It became a best seller in nine countries with four sequels that sold so well that in 1978, Conran's accountant suggested she move to the tax-free principality on the Riviera, Monte Carlo.

Conran seems defensive of her reputation as a writer. She asks her interviewer if he liked her book and, receiving a lukewarm answer, suggests his wife will appreciate it more. She didn't like it either, says the interviewer. In her whiskey voice, Conran expresses surprise.

"Women have been my subjects for 20 years," says Conran, "and I know what I'm doing. I studied them as if they were a different species . . . and I'm very fond of them. I know my audience very well. One-third of my audience is men, who read me to find out what women are thinking about."

Conran began writing the outline for "Lace" in the Beverly Hills Hotel on her birthday three years ago this month. She was tired of researching books, and thought she'd try her hand at a novel for a change of pace. First came the title.

"I wanted something that would have a different connotation to everyone and would be short," she recalls. Throughout the book there is plenty of lace, especially as in lingerie.

"Then," says Conran, "I invented four characters, and then I had to invent a solution."

The New York publisher who handled "Superwoman" suggested Conran show her 26-page synopsis to lawyer and super-agent Morton Janklow, whose clients include Judith Kranz, William Safire, Marvin Kalb and Sidney Sheldon. Late in 1979, Conran met with Janklow.

"What I saw," recalls Janklow, "was very well thought-out. She had a powerful commercial proposal--if she could write to that proposal. 'Listen,' I told her, 'I can sell from this proposal but if you're not under any pressure, I'd rather sit tight until you write the book.' I waited 18 months. I'd call her up and schmooze about it every once in a while."

Conran already knew about Europe, so the London, Paris and French countryside scenes were no problem. Her knowledge of this country was courtesy of a five-week Greyhound tour she took "down the left-hand side, across the Gulf of Mexico and up the right side with my American Express card stuck in my bra."

And her four characters who went to school together all achieved success in fields which Conran herself had worked: public relations, interior design, journalism and fashion. Conran says she took descriptions of her characters to a psychiatrist to "check them for motivation and plausibility."

But what of the steamy sex scenes? Conran shrugs. And the Middle Eastern potentate who introduces a goldfish into his lovemaking? "A sort of kindergarten Japanese perversion," she says.

Conran delivered all 1,300 manuscript pages of "Lace" to Janklow last summer. He proposed the book to five publishers, and within a week, according to Janklow, all were "wildly interested." Michael Korda won the book with his minimum offer of $750,000, a total, Janklow says, Simon & Schuster earned back in foreign and dramatic rights before the book even went on sale last month.

When Conran visited Washington a couple of months ago to begin her promotion tour, she asked a buyer for Crown Books what she thought of her racy romance.

"Trashy," said the book chain executive matter-of-factly, "but it'll sell."

Right on the money.