Fox, which plans to air an interview with Simpson Nov. 27 and 29, said Simpson describes how he would have committed the 1994 slayings of his ex-wife, Nicole Brown Simpson, and her friend Ronald Goldman, "if he were the one responsible."
"I'm not surprised at anything that piece of garbage — that murderer — would do," Goldman's father, Fred Goldman, said of O.J. Simpson Wednesday.
Denise Brown, sister of Nicole Brown Simpson, lashed out at publisher Judith Regan of ReganBooks for "promoting the wrongdoing of criminals" and commercializing abuse. The book goes on sale Nov. 30.
She added: "It's unfortunate that Simpson has decided to awaken a nightmare that we have painfully endured and worked so hard to move beyond."
Like Brown, Fred Goldman voiced disgust toward Regan and Fox. "They themselves have fallen to an unbelievably low standard," Goldman told CBS News.
Simpson was acquitted in 1995 of murder after a trial that became an instant cultural flashpoint and a source of racial tension. The former football star was later found liable for the deaths in a wrongful-death suit filed by the Goldman family. In the years since, he has been mocked relentlessly by late-night comedians, particularly for his vow to hunt down the real killers.
Watch Fred Goldman's reaction to O.J. Simpson's book.
Regan refused to say what Simpson is being paid for the book but said he came to her with the idea.
"This is an historic case, and," Regan told The Associated Press.
Simpson has failed to pay the $33.5 million judgment against him in the civil case. His NFL pension and his Florida home cannot legally be seized. He and the families of the victims have wrangled over the money in court for years.
In recent years, Simpson has made money signing autographs at trade shows. But according to the National Enquirer, he's being paid $3.5 million for his story, reported CBS News correspondent Hattie Kaufman.
The victim' families could go after the proceeds from the book's sales to pay off the judgment. But one legal analyst said there are ways to get around that requirement — such as having proceeds not go directly to Simpson.
"Clever lawyering can get you a long way," said Laurie Levenson, a Loyola University law school professor and former federal prosecutor who has followed the case closely.
Levenson noted that the criminal justice system's protection against double jeopardy — being tried for the same crime twice — means Simpson's book, explosive as it may be, should not expose him to any new legal danger. She added that Simpson could create an extra layer of insulation from any legal worries by presenting the story hypothetically.
"He can write pretty much whatever he wants," Levenson said. "Unless he's confessing to killing somebody else, he can probably do this with impunity."
Goldman's sister, Kim Goldman, told CNN: "He's snubbing his finger to the system, to the community again. He's telling us one more time, 'I'm gonna continue to get away with killing your family members and I'm not gonna honor the judgment and look at me, ha, ha, ha."'
Simpson did not return numerous calls for comment. Simpson's own attorney Yale Galanter said he did not know about the book or the interview until this week.
"The book was not done through our office," Galanter said. "I did not have anything to do with the negotiations of the book. This was strictly done between O.J. and others."
He said there is "only one chapter that deals with their deaths and that chapter, in my understanding, has a disclaimer that it's complete fiction."
On Amazon.com on Wednesday, the 240-page book was being offered for $16.47. An image of the cover featured Simpson's face and the title "If I Did It," with "If" highlighted in white and the other letters in red.
Other publishers and publishing industry observers practically fell over each other to criticize ReganBooks, an imprint of HarperCollins Publishers, and Simpson.
Otto Penzler, who runs Otto Penzler Books, a crime imprint of Harcourt, said he would have a moral problem with "carrying a book like that and enriching this lowlife in any way."
"If I were betting, I would say the book won't sell," he said. "I think people are so disgusted with this guy that they're having the same feeling I do."
ReganBooks has gained a reputation for publishing some less-than-highbrow material, including Jose Canseco's "Juiced," billed as a tell-all on steroids in baseball, and books about the slaying of Laci Peterson.
Patricia Schroeder, president and chief executive of the American Association of Publishers, described the developments as sickening.
"But I think it's going to stir an awful lot of debate and make the culture take a real look at itself, and that may not be unhealthy," she said.
Indeed, one thing that seemed certain was that the book and interview — which Fox will air at the end of the crucial sweeps month — were bound to generate a torrent of publicity.
Shari Anne Brill, a television analyst for the Madison Avenue firm Carat USA, predicted public interest would rival that of the 2003 interview with Michael Jackson, seen by 27 million people in 2003.
At least one other network, NBC, said it had been approached to air the special but declined the offer.
"This is not a project appropriate for our network," said Rebecca Marks, a spokeswoman for the entertainment division of NBC, a network that once employed Simpson as a football analyst.
CBS said it was unaware of any pitch for the project. ABC did not immediately return a call for comment.