A Mexican millionaire has electrified the political and social scene here with a cutting expose in her versions of the secret goings-on in the inner circles of Mexico's wealthiest and most powerful family.

Her book, "Our Group," written surreptitiously at night, offers the sort of blazing indescretions about Mexico's mighty of which the public rarely gets a glimpse: political power games played all the way up to the presidencey, shady business deals, tales of corruption, murder and high living, savored with rich detail of illicit love affairs.

The unexpected author is a mother of eight and grandmother of 17. Irma Salinas, 57, and her kin about whom she writes are the industrial barons of Monterrey, some members of the Garza Sada family, whose name is as familiar in Mexico as Rockefeller in the United States.

Salinas' excursions into the family closets and the dirty linen, even the skeletons that she claims, have all the makings of a best-seller - a best-seller, that is, had the book not disappeared.

Just days before publication, 30 state police burst into the printer's shop and took off with 17,000 copies, the plates, the manuscript and the printer himself. Then police raided the author's home, seized her private copies and charged the book was "vulgar" and "libelous."

Some family members had, according to well-placed sources, got hold of several chapters and without bothering to go to court had allegedly used their influence with the police. Even the local federal attorney's office refuse to register Salinas' denuciations. Under cnsiderable pressure, a weekly magazine bowed out of serializing the book.

The police action has caused and out-cry in the press, which argues that while the book may not be respectable, its illegal confiscation only underlunes the author's premise: the power of the Garza Sada family. Salinas did not leave her home for three weeks. She alleges she was surrounded by cars full of gunmen and was swamped with telephone threats.

Surviving copies of "Our Group" inevitably have becomes hot property: Some have shown up on the black market fetching as much as $130 apiece, spotty photostats have circulated like secret documents.

The authorities, not given to public explanations, have said nothing about the book's confiscation or its contents, the most controversial part of which relates to the murder of the family patriarch.

The book stunningly claims that the 1973 assassination of family patriarch Eugenio Garza Sada was not the work of leftist guerrillas, as the country had been led to believe, but had been ordere by some prominent members of the family.

Don Eugenio, as he was widely known, was the man mainly respinsible for turning a family beer brewery and glass factory into what is now a $1.5-billion industrial concern taking Monterrey from a small desert town at the turn of the century to one of Latin America's most important industrial centers. The huge conglomerates now embrace chemical, steel and cement Plants, textiles, television, two nationwide banking chains and employ 80,000 people.

With most shares held among the myriad Garza Sada relatives and descendants, composition of ownership of the "Monterrey Group," as it is known, remained hidden. But as successive Mexican administrations cried for rapid industrialization and the "group" with government support, expanded it became the nation's most powerful political lobby, speaking for the moneyed classes of Mexico.

It was therefoe not surprising that presidential candidate Luis Echeverria had close contact with Don Eugenio or, Salinas recounts, that he alerted him in the late '60s that the social tensions and the recent army repression demanded a tactical change. His government, Echeverria is reported to have said, must adopt the apparent position of defending the masses and turning against the entrepreneurs. Don Eugenio, Salinas writes, understood and agreed to inform the other industrial captains.

Once Echeverria became president, she goes on, he began to press Don Eugenio to sell HYSA, the "group's "most powerful industry, producing 1.3 million tons of steel per year. This would bring Mexico's basic industries - oil and electricity were already nationalized - under state control and would represent a major patriolic achievement for Echeverria.

Again, Don Eugenio agreed and, in so doing caused a great uproar within the family. There had been frequent passionate in-fighting behind the facade of family unity, now it turned into a rift. Finally, the author alleges, the decision was taken by some family members to kill the 81-year-old man because he was clearly losing his mind. The Turning Point

The events that followed are still shrouded in mystery. Don Eugenio was shot in his car in the early morning of Sept 17, according to police, in a leftist kidnap attempt, according to Salinas' allegations, by hired gunmen. The two men did the firing were found dead later that day, their bodies thrown behind a cemetery Salinas says she has had access to court documents, judges and cabinet ministers and writes of contradictions between statements of police, witnesses and the accused.

Five leftist guerrillas, charged with "plotting the kidnapping," have been in Monterrey jail for 4 1/2 years without trial or sentence. Relatives claim that they have been "tortured and forced to confess."

HYLSA, the steel works, was not sold to the government; the family rift became clear as the conglomerate split into several groups.

Whatever the truth about the Garza Sada killing, it was a turning point in the Echeverria regime. Around 150,000 people attended Don Eugenia's funeral, which was a virtual declaration of war against Echeverria by a private sector already suspicious of the mildly reformist president and his inflationary spending. Echeverria is now accused of fostering letfism and chaos.

Yet if it were proved that the respected dean of Mexican business was killed by the right, this would throw a very different light on the turmoil of Echeverria's last years in office.

Echeverria's supporters, in turn, charged the "Monterrey Group" had a guiding hand in the rapid weakening of the economy by starting the massive flight of capital - $4 left Mexico within a short period - and precipitating the peso devaluation. The "group" was generally regarded as responsible for initiating a whispering campaign that Echeverria was planning a coup to remain in power which turned his final months into a period of near-national hysteria. Behind the Facade

Aside from the sober, political sections of the book, Salinas uses a sharp tongue and a very personal style to describe the exclusive, tight knit family which she entered by marrying Roberto G. Sada, grandson of one of the founders of the Garza Sada empire. (Sada died a few years ago is a diving accident in the Carribean.) Although she is a millionaire in her own right (having inherited her father's department store chain) she was deeply shocked to find out what life was like with "the untouchable Monterrey Royalty," who have politicians at their call.

To enter the "Monterrey Group's" social circuit, the best way, she writes, "is to be an industrialist. If not, a banker; a third way is to be a wealthy merchant, although they are up to a point despised; the last, and a good method, is of course to have an affair with a woman of the 'group.'"

"Our Group" gives a rare view behind the respectable facade of Mexican society where some characters with private chapels in their homes also, the author alleges, lead hedonist lives, have long affairs and illegitimate children.

"The great absolutely pagan parties they gave could always keep the appearance of charity thanks to the ingenuous Franciscan monks who would attach their names to such events," Salinas writes.

When the extramarital affair of one of the Garza Sada women had to be broken off because it was causing too much scandal, Salinas claims that the lover was dismissed with generous compensation. "So if the secret, passionate encounters came to a halt," she writes, "his tequila business received powerful injections of capital."

Another relative, also a member of Monterrey society, she describes as a typical Mexican macho always ready to show off his latest-model sports car or newest mistress. She alleges, "But he always kept his wife pregnant as a fruit-bearing tree, inside her home, covered up to her nose so that no one else would desire her." A Virtual Outcast

Last year, the twice-widowed Salinas caused a minor storm with her autobiography in which she tells of her own love affairs, something unheard of in polite society here. "It was all part of a plan," she says. "I had to undress myself first, otherwise 'Our Group' could be discredited by people trying to expose me."

Like so many affluent women in Mexico, she always had a low profit, moved in the exclusive circles of the "oligrarchs," worked on charity projects and traveled abroad.

Now, she says, she is a virtual outcast in Monterrey where friends and relatives either completely avoid her or send notes saying , "Congratulations, but you must understand I can't be seen talking to you." Two of her eight children, directors in family companies, have been forced to resign, she claims, while one of her researchers has been beaten up and threatened.

Expecting problems, Salinas says she decided to write the book secretly. "With my eight children, and 17 grandchildren coming in and out all the time, it was impossible to go around hiding my papers everytime the bell rang," she says. "My children were furious about the first book. So I wrote every night from 9 till 6 the next morning. Her secretary and researchers would continue the work in the daytime."It took 3 1/2 months. I'm still exhausted from it."

If she knew there would be so much trouble, why did she write the book?

"It's something I had to do," says Salinas. And she claims further - "There are innocent people in jail blamed for Don Eugenio's assassination. My own children were cheated out of their inheritance. I've begun to understand the abuse of power and the alienation that comes from the need to gain more and more power through money. The Garza Sada family has created a state within a state. It's very dangerous. That's why they need to be unmasked." 'I Can Fight'

Her conscience, she says, does not bother her. "People have to know about those that have the economic power in Mexico. How they act, what sort of judgment they have. And anyway, all the private things I write, everyone knows about in Monterrey - employes, wives, servants. But this is how things are done high up. Everyone plays at convincing themselves that no one knows what everyone knows." The chapters full of erotic scenes she says have a purpose: "I know I'm ridiculed for that because I'm a grandmother. But I want a lot of ordinary people to read this book. Not just a few insiders, the politicians or the businessmen."

No member of the Garza Sada family has made statements to the press. Privately, a family member told a reporter that Salinas had caused them "terrible headaches and had to be stopped even by force. She is going mad, and the worst part is that she has money so it is difficult to stop her."

Salinas suspects that her relatives will want to ruin her economically, although "that will take them a while. But they have already managed to get an airport which I own closed down, "she claims. "It's too close to a school, I am suddenly told by the authorities. Except that school has been there for 10 years. They know of course as long as I have money. I can fight and write and pay lawyers."

Salinas says nothing can hold her back. "They also said that Martha Mitchell was mad. I will soon write another book showing the moral decay of our ruling class."