''News that the K.G.B. was attempting to plant false stories in the African press portraying Dr. King as an 'Uncle Tom,' at the very time when Dr. King was harshly attacking Johnson's conduct of the Vietnam War indicates that American police agencies were not the only Keystone Kops active in the 1960's,'' said David J. Garrow, a historian at Emory University and the author of ''The F.B.I. and Martin Luther King Jr.''

Mr. Mitrokhin was a K.G.B. archivistin charge of managing many of the spy service's secret files until he retired in 1984. When he arrived in Britain in 1992 and sought out British intelligence, he brought with him a huge cache of notes that he said he had taken based on those files, and turned them over.

The Mitrokhin files, which the British considered reliable enough to share with the C.I.A. and F.B.I., have offered Western intelligence and law enforcement officials a treasure trove of historical information about K.G.B. operations around the world.

And while the archives quoted in the book contain only limited information about Soviet espionage cases, they have already helped identify some spies. In the United States, for instance, the book reveals that the Mitrokhin files helped lead the F.B.I. to Robert Lipka, a former code-clerk at the National Security Agency, who worked as a Soviet mole in the 1960's. Mr. Lipka was arrested in 1996 and pleaded guilty to conspiracy to commit espionage. Information about other open spy cases contained in the archives were withheld from the book, including the case of a former State Department official, Felix S. Bloch, who was suspended in 1989 and resigned in 1990 but was never charged or arrested.

Mr. Mitrokhin first tried to defect to the United States but received a lukewarm reception from a C.I.A. officer when he approached the agency in a Baltic country soon after the collapse of the Soviet Union.

Officials say that the C.I.A.'s Soviet/East European Division had decided that the K.G.B. was no longer a threat and had instituted a controversial policy that led C.I.A. officers in the field to turn away many defectors. Paul Redmond, who was then the C.I.A.'s deputy chief of counterintelligence, said in an interview that he sought to take over the Mitrokhin case after other officials had failed to show interest, but by then Mr. Mitrokhin had turned to the British.

Mr. Redmond now argues that the C.I.A.'s diffident handling of Mr. Mitrokhin's efforts underscored a larger problem, which was that the C.I.A. decided ''naively'' after the collapse of the Soviet Union to scale back its espionage operations against Moscow. ABC News reported on this controversy on Thursday.