I AM grateful for the positive review Steven Mufson gave my book Apartheid's Rebels: Inside South Africa's Hidden War (Book World, December 13), but I feel it necessary to clarify an important point.

Mr. Mufson states that I describe key legal anti-apartheid groups in South Africa as "controlled" by the outlawed African National Congress. The ANC is indeed the predominant resistance movement in South Africa -- yet I have not argued that it exercises anything as strong as "control" over allied black groups such as the United Democratic Front or the Congress of South African Trades Unions. On page 114 of my book I summarize the complicated and frequently strained relations between the parties: "Neither the UDF nor COSATU, nor most of their affiliates, were controlled by the ANC. While they clearly cooperated with, consulted and often followed the Congress, even recognized it as preeminent in the nationalist struggle, each had developed separate agendas for the nation's future."

As Mr. Mufson and I agree, war is spreading through South Africa, even if it is being masked by censorship measures. As the violence escalates, observers must be accurate in assessing the strengths of both sides. Apartheid's Rebels traces the growing power of the ANC -- but not without also exposing the group's persistent weaknesses. Stephen M. Davis Washington, D.C.

PHILIP GEYELIN began his review of Phillip Knightley and Caroline Kennedy's An Affair of State: The Profumo Case and the Framing of Stephen Ward (Book World, December 20) by trying to persuade us that reaction to what he called a "British sex scandal" outscored American reaction to Gary Hart's adventures.

The British case was surely not primarily a sex scandal. The chains that wrapped up John Profumo's career, and caused Harold Macmillan's government to lurch, were forged out of national security and espionage. The links that connected Mr. Profumo to other participants in the affair were sexual, certainly. Consider, though, the facts that John Profumo was Her Majesty's Secretary of State for War, and Christine Keeler's links included an attache' at the Soviet Embassy who had KGB connections.

The magnitude of the political consequences of this case had little to do with the British attitude toward sex. It was truly "an affair of state." Trevor Smith Kensington, Md.

KAREN DeYOUNG'S review of Robert Pastor's Condemned to Repetition: The United States and Nicaragua (Book World, November 8) was a travesty and a slick effort at rewriting history, as well as an attempt to blame President Reagan and protect former president Carter.

U.S. policy as regards Somoza was cold and calculating once the Carter administration was in office. Long before the Sandinistas were a real force, decisions were made in Washington to cut off military supplies to Somoza as well as spare parts for the American equipment he did have. Then, when the Sandinistas began to receive men and guns from communist nations around the world, we even stopped Israel from supplying the weapons and ammunition Somoza needed to fight.

Karen DeYoung knew -- and everyone concerned knew -- that there was no "third force." The State Department was aware of what was going on and admitted at a briefing in 1977 that the Sandinistas had all the guns and were going to take over when Somoza fell. President Carter was also well informed of the actual situation. A letter was sent to him by 70 members of Congress that told him what was bound to happen and this was long before Somoza fell. Carter was reportedly furious because one of the signers of this letter was now-Speaker Jim Wright.

People like Karen DeYoung and Pastor made a career of dumping on Somoza because he was the most consistent anticommunist ruler in Central America at that time. True, under Somoza, there was not 100-percent democracy, but La Prensa had no difficulty publishing bitter criticism of Somoza, a parliament functioned, albeit dominated by Somoza adherents, and church and property rights were not under assault as they are now. Nicaragua, moreover, enjoyed one of the highest standards of living in Central America at that time. Robert Pastor was never interested in a "third force." His only interest was in getting rid of Somoza. He succeeded and we now have a little Castro named Ortega on our hands in Central America. Frederic N. Smith Arlington, Va. Queries A BIOGRAPHY is to be published by Simon & Schuster of the late James J. Angleton, the CIA's former chief of counterintelligence (1954-74). I would appreciate hearing from anyone with information, personal reminiscences, letters, documents, photographs or anecdotes about his intelligence career in London, Rome and Washington. Please write in care of my researcher Jeff Goldberg, 1410 26th St. #2, Washington, D.C. 20007, or to me directly in care of BBC Television, London W12 7RJ, England. Tom Mangold London, England

FOR PERMISSION to quote from unpublished writings, I am seeking the heirs of Forbes Watson (1880-1960), an art critic and for many years a resident of Washington, D.C. I would appreciate hearing from anyone with the names and addresses of the executors of his estate or that of his wife, Nan Watson (1876-1966). Avis Berman 116 W. 75th St. #3A New York, N.Y. 10023

I SEEK the location of diaries, letters and photographs of men who served with the American cavalry regiments stationed in the Philippines during their occupation by the United States. Documents from wives and children of Army personnel would be welcome as well. Bradford K. Klein 902 Old Riverway Ct. Sterling, Va. 22170