For nearly three decades, audiences in all parts of the world, unfoolable in their demands for quality, have known that Joan Baez can sing. Now comes evidence that she also has a prose voice of rare timbre. Her memoir, "And a Voice to Sing With," is being published this month.

The autobiography market is currently heavy. Tip O'Neill, the former statesman now grubbing for easy money in beer commercials and credit card ads, is competing with Vanna White for the memoir dollar, with both tales having all the bite of loose dentures.

Baez, by contrast, has a book worth devouring. Line for line, it is a lovingly limned accounting of the people and ideals she has aligned her talents with. Baez has always been more than a stage presence or pure soprano voice waxed on some 30 albums. She has caught the slants of fame's sunshine and deflected them to purposes larger than herself. No loss of contemporaneity has occurred. The music business -- starting in 1962 when at age 21 she was on Time's cover as the peace movement's barefoot madonna -- led her into life's only soul-satisfying calling: easing the suffering of others.

"I was born gifted," she writes. "I can speak of my gifts with little or no modesty, but with tremendous gratitude, precisely because they are gifts and not things which I created, or actions about which I might be proud. My greatest gift, given to me by forces which confound genetics, environment, race, or ambition, is a singing voice. My second gift ... is the desire to share the voice, and the bounties it has heaped upon me, with others."

How many performing artists have been as inviolately steadfast as Baez to one creed and one commitment? "I have been true to the principles of nonviolence," she writes, "developing a stronger and stronger aversion to the ideologies of both the far right and the far left and a deeper sense of rage and sorrow over the suffering they continue to produce all over the world."

Baez, now 46, is the second of Joan and Albert Baez's three daughters. The family, still close and living within range of each other in Northern California, went to Quaker meetings. There the father, a physicist who had been offered but turned down military research, absorbed the meeting-place silence and "became a pacifist. Rather than get rich in defense work, he would become a professor. We would never have all the fine and useless things girls want when they are growing up. Instead we would have a father with a clear conscience. Decency would be his legacy to us."

As an eleventh-grader in Palo Alto (Calif.) High School, Baez went to a conference organized by the American Friends Service Committee. Martin Luther King Jr., then 27, told the high school students about organizing a nonviolent revolution: "When he finished his speech, I was on my feet, cheering and crying. King was giving shape and a name to my passionate but ill-articulated beliefs. Perhaps it was the fact of an actual movement taking place, as opposed to the scantily attended demonstrations I had known to date, which gave me the exhilarating sense of 'going somewhere' in my pacifism."

The somewhere would be everywhere. Baez would sing for King in Mississippi, Lech Walesa in Poland, bomb victims in Hiroshima, political prisoners in Russia, mothers of the disappeared in Argentina, frightened families in Belfast and refugees in Laos. At Christmas in Hanoi in 1972, when Nixon and Kissinger ordered modern warfare's heaviest bombing, Baez survived death in underground shelters. She sang in Hanoi for "all the Vietnamese and Americans who had died in the war, and then to all the men who had refused to fight it from the beginning, and finally to those who had quit fighting when they had become disillusioned (or, illuminated)."

Behind this ardent filigree of commitment, Baez was earning money -- and giving most of it away -- from U.S. and European concert tours. College kids now come up to her and say, "My parents have your albums in the basement," while upstairs the children listen to Madonna. Baez writes poignantly of the latter: "What will happen to you, baby child, when the spotlights dim and morning sunlight finds your eyes red from weeping. Come and see this old madonna, who will tenderly serve you jasmine tea and say quietly in response to the unformed questions struggling up from the ashes of your fiery young life, 'I understand, sweetie, I understand.' But for now, in the diamond glow of success, dance and sing and bump and grind in your jangling glitz necklaces and skintight mini bun-huggers ... Someday maybe those handsome Playboy tits of yours will find a more earthly purpose and you, a more fulfilling life ... Trips to the supermarket will not be easy at first."

Baez, dotingly happy in the co-raising of a teen-age son, has done the family shopping and survived. She writes jauntily of the adjustments. To the politically atrophied folkies who want to stay stuck in the '60s and hear from her only the faded anthems, she laughs and promises someday to do an album of the oldies. But not now. Nostalgia is for those afraid of the future, a fear Baez has never had.