IN THE SPRING of 1968, studying slavery for his doctorate in American history, Terry Alford was browsing in the archives of the Natchez, Mississppi courthouse when a piece of yellowed paper, stuck between the pages of a volume of 1828 deeds caught his eye. It was a letter, dated January 12, 1828, signed by Henry Clay, who was secretary of state at the time, and concerned the freeing of a slave who was said to be a "Moorish" prince. As things turned out, the discovery disrupted the next eight years of Alford's life and resulted in his book, Prince Among Slaves , which Harcourt Brace Jovanovich will publish November 29.

The story of Ibrahima, the slave prince, is "so romantic you wouldn't dare put it in fiction," Alford says. "And oddly enough, there is no record of him in black history books." He was born in 1762 in Timbo (now part of the African republica of Guinea) where his father, Sori, was king, and was educated in Timbuktu, a center of Moslem culture at the time. When he was 19, he and his father came to the resuce of Dr. John Coates Cox, a one-eyed Irish surgeon and adventurer, who was lost on a hunting expedition.

Seven years later, Ibrahima was captured, sold as a slave, and wound up on a Natchez plantation. Fifteen years after that, as he was selling sweet potatoes for his master at a country market, lo, he encountered the man whose life he had saved. The doctor recognized him, and repaid the favor by helping to set Ibrahima free . . . which led to the letter from Henry Clay.

Alford learned the first outlines of the story from Miss Mary Postlethwaite, a Natchez antiquarian and repository of local lore, and the search for Ibrahima's roots was on. "I've been working almost as long as Alex Haley," Alford says, "but my problems were a different sort," Ibrahima's story was famous in his day. There were newspaper accounts of his public appearances and several locally published books about him in what Alford calls the "moonlight and magnolia" school of writing. The challenge was to cut through the legends and establish the facts.

The trail led to New York, New Haven, Boston, Philadelphia, New Orleans, Washington, England, France and Africa. In the Boston Public Library, he found a description of Ibrahima in a letter from a woman to a friend. He found three manuscripts in Ibrahima's own hand (fragments from the Koran and sketchy biographical notes - in Arabic). One was in the library of the American Philosophical Society in Philadelphia; one at Yale, and one in the Manuscript Division of the Library of Congress. In the Prints Division there, he found a lithograph of the prince. (The prince is indexed, if at all, as Ibrahima, or Prince, which was his slave, name, or Abdul, another of his given names, or African Prince, or King of Timbuktu.)

He found the name of the slave ship that carried Ibrahima to New Orleans in the Public Record Office in London. At the Institut de l-Afrique Noire in Dakar he found a vast collection of oral histories relating to Ibrahima's birthplace, which is in Futa Jalon, a mountainous area of Guinea, inhabited by the Fula tribe - the ruling minority in Guinea until its independence from France. He was never granted a visa to go to Senegal, abounds in Fula historians in exile, who were able to furnish some missing details. They still tell stories of Ibrahima's father and some of his many brothers in Futa Jalon, but Ihrahima had dropped off the charts.

"What impressed me most, as I retraced Ibrahima's life," Alford says, "was the way he conducted himself and how he withstood adversity. He had a remarkable personality."

By 1970, Alford and his wife Jeanette moved near Washington, to be closer to the research sources for the book. When Jeanette wasn't taking notes, she took odd jobs to keep the family afloat. She worked with computers at George Washington University, was a research psychologist at the Census Bureau, and was "one of Washington's first female bicycle messengers." She also restored prints at Georgetown's Old Print Gallery, taught kindergarten, ran a "by appointment only" rare manuscript business that she and her husband had started as a hobby, and bore two children. Amy is now six and Nathan arrived a month ago. The Berenson Facade

MERYLE SECREST, now married to Tom Beveridge, a composer, observes that "vistas of only having to earn half an income bring on possibilities of writing books." (Her first, Between Me and Life , a biography of the artist Romaine Brooks, was published by Doubleday in 1974). Now, with a contract from Holt Rinehart in this country and Weidenfeld and Nicolson in England, she is working on Being Bernand Berenson , a biography of Himself. "Professor Ernest Samuels, biographer of Henry Adams has been hand-picker by Berenson's secretary and heir to write his 'official' biography, so I am using the journalist technique - writing from outside the story. Either it will be useless or a tour de force ," she says. "But you'd be surprised how many people knew Berenson. I've done interviews in Italy and London. There are thousands of Berenson's letters all over the world and his family has shown me stacks of stuff."

"I'm interested in his character." The legendary Berenson "is a facade. Who was the man? His father was a Lithuanian Jew, who got out before the pogroms. He was a brilliant scholar at Harvard. He has a brilliant heritage - East European Jew and proper Bostonian . . ."

Secrest says he is "fascinated by biography. I've been in journalism since I was 19, and have done an awful lot of profiles. And in book writing you don't have to bow to the whims of copy editors."