The car bombing that killed former Chilean ambassador Orlando Letelier and his associate Ronni Moffitt three years ago is about to become the subject of a book and a major motion picture.

Eugene M. Propper, 32, the prosecutor in the Letelier case which went to trial last winter, and Taylor Branch, a Washington free-lance writer, will produce the book. Branch said yesterday that he and Propper "hope to sign something in the next few weeks" with Viking Press. The sum for the hardcover book deal will be $175,000, according to industry sources.

"All the terms have had hands shaken on them," said Viking editor Amanda Vaill. "We're in the process of negotiating the last little nits."

In Los Angeles, Mark Rosenberg -- Warner Bros. vice president for productions -- said yesterday that he has "closed the deal" for the film rights from Propper and Branch. And the company expects that the arrangements for the entire movie package -- totalling several million dollars, according to an industry source, will be concluded "in the next day or so."

Rosenberg called the story of the Letelier assassination plot "the penultimate American thriller," and said that the movie would be directed by Sydney Pollack and the screenplay -- which would be finished some time next year -- wouod be written by David Rayfiel.

Pollack directed "They Shoot Horses, Don't They?", "Three Days of the Condor" and "The Way We Were."

"I don't know that the name Letelier rings bells in the minds of Kansas farmers," said Rosenberg. "That's not the point. It will be an interesting story." But, he said, "I do not want to give the impression that anyone is trying to profit in any deleterious way from the murders of Letelier and Moffitt."

"I would be offended if anyone thought this was going to be a cheap and exploitative book," said Branch, who said he has long been interested in the case. Propper was not available for comment.

But Michael Moffitt, whose wife Ronni was killed in the car bombing, said he was "very upset" by the book venture.

"I think it would be a little silly for the former U.S. Attorney to talk about the case before it's over," said Moffitt, referring to Propper. Currently, the U.S. government has asked Chile extradite several of their government officials implicated in the death of Letelier. "This could just become a commercial spectacle instead of the hideous crimes that they are."

Moffitt said he, himself, has had several inquiries from book and movie companies. "I've talked with a lawyer about possibly working out an arrangement with a publishing company to have the story told by someone. But where we could have some control.

"I prefer, if anything, that this really tell people what happened -- not some crazy commercial thing."

Branch said that "it's been in my mind to possibly do a book all along. It was a remote possibility."

Branch wrote extensively about spies and Cubans for Harpers's magazine in the past. He said he went to Venezuela about a month after the bombing. "I knew Cubans who had talked of using the kind of bomb that killed Letelier. And I knew Cubans who talked about people with ties to the Pinochet regime in Chile. I was into it enough to fly off to Venezuela on my own. I thought I was ahead of the FBI."

Commenting on Moffitt's displeasure at the book venture, Branch said, "Michael's never said anything to me about it, and Isabel Letelier has been encouraging. I asked her at a party one time what she thought about the possibility of doing a book and she said she hoped I would do it. She hoped I could get as much out in the open as possible."

Both Letelier and Michael Moffitt are affiliated with the Institute for Policy Studies, the liberal think tank for which both Orlando Letelier and Ronni Moffitt worked at the time of their deaths.

In the federal court trial, three anti-Castro Cubans and a former Chilean secret police agent were convicted -- and two received life sentences -- for their involvement in the death of Letelier, a leader of the exile opposition to Chile's military regime. At sentencing, the judge said that Letelier had been "the victim of a brutal international conspiracy." The trial included testimony from the Chilean agent, Michael V. Townley, who matter-of-factly described for a stunned courtroom how he planted the bomb in Letelier's car.

Rosenberg, who said he went to high school with Ronni Karpen Moffitt in Passaic, N.J., said he was "personally committed to the project."

Viking's Vaill, who will be the editor for the book project, said, "Our feeling is we have a story that has many levels. It's a story that deals with Washington, foreign relations, terrorism, and the mechanics of a court conviction."

The book will also unveil "new and surprising revelations about the case and the people involved in it," said Vaill. "It's not necessarily shocking information, but it will have interesting narrative value. I think I can pretty safely say all the evidence that ought to be dealt with by the courts is in and being dealt with."

But yesterday afternoon, Isabel Letelier's lawyer, Michall Tigar, filed a notice of deposition for Eugene M. Propper in federal court, in connection with the civil suit of Letelier against the Republic of Chile and the defendents who were tried last winter in the Letelier trial.

"We think Propper knows something about the case beyond what we already know," said John Privitera, a lawyer who works with Tigar. Privitera said the filing of the notice was prompted by Propper's resignation from U.S. Attorney's office last month and by a notice in the September issue of "The American Lawyer" that said Propper was trying to write a book about the case.