Less than 24 hours after I testified before a grand jury investigating those murders," Florence Mars recalls, "the Klan initiated a campaign to ruin me."

The year was 1964, and Mars and her 67-year-old aunt, Ellen Spendrup, had broken the code of the white South. They had assisted the FBI in its investigation of the killing of three young civil rights workers in Philadelphia, Miss.

"The Klan boycotted my stockyard business," she says, "and forced me to sell. The community began to regard me as a 'Communist agitator.' Finally their propaganda succeeded in separating me from the fellowship of the First Methodist Church."

Mars, 54, whose family dates back four generations in Neshoba County, found herself torn between principle and loyalty to her neighbors. Why did she take such a risk and testify? "I wanted the community to see that it should oppose murder no matter who committed it." (Eventually 18 men, including the sheriff and a deputy, were tried in connection with the murders, which caused a worldwide furor.)

To try to understand what was happening to her and her town, Mars started writing "what I called my thesis." Last August, after 14 publishers had turned the manuscript down as not commercial, Louisiana State University Press published it as Witness in Philadelphia. Now in its second printing, the book has received widely favorable comment and is a Book-of-the-Month Club alternate (although some white Philadelphians complained, "I don't see why Florence has to stir all that up again." The town in recent years has integrated its schools peacefully and improved black housing and job opportunities).

Although U.N. Ambassador Andrew Young wrote of her book, "Florence Mars is both a symbol and a living practitioner of the belief that people who are willing to challenge injustice can prevail," she genuinely does not see herself as a heroine. Instead she pays tribute to her spunky aunt and to blacks like Bud Cole, victim of a near-fatal Klan beating, and 84-year-old Lillie Jones, who registered to vote in 1965. "It was the first time in her life Miss Lil knowingly did something white folks wouldn't like," Mars says.

She took a copy of Witness to the Jones house. "One week later," Mars says, "Miss Lil asked me to come over and get some pound cake. When I arrived she said about my book, 'It just can't be beat for telling the truth.' "