"According to the FBI, Mitrokhin’s documents are 'the most complete and extensive intelligence ever received from any source.' Perhaps the most significant prize for the Western intelligence community are the documents that contain the real names and identities of thousands of foreign agents the KGB recruited and kept under deep cover abroad—a rosetta stone for the spy world...Many of the documents remain classified.”
[Stromberg, Stephen W. "Documenting the KGB". Oxonian Review of Books. Winter 2005]
The Mitrokhin Archive became public on 9-11-99 when serialization of the Mitrokhin Archive by Cambridge historian Christopher Andrew and the late KGB archivist Vasili Mitrokhin [London Telegraph obituary, 2-25-04] began in the The Times.
"Mitrokhin's archive consisted of material culled from tens of thousands of top-secret KGB files, which he had laboriously copied down over 12 years and hidden in tins and milk crates underneath his dacha. It contained detailed records of every operation the KGB had mounted from its inception in 1917 to Mitrokhin's retirement in 1984, demonstrating the extent to which the KGB had successfully infiltrated the West and the way in which it had oppressed the Russian people....The Mitrokhin archive led to resignations, arrests and a few prosecutions around the world, though there are believed to be about 300 Soviet sources still living in Britain and America who have not yet been publicly identified." [See full text: London Telegraph Obituary, 2-25-04]
The former head of the History department at Cambridge University, Christopher Andrew, and the former KGB archivist, Vasili Mitrokhin, wrote six books based on archival materials that the disaffected KGB employee noted or transcribed from the KGB archives.
"The Mitrokhin Archive" refers to the handwritten notes that Mitrokhin made over the course of 30 years about the contents of secret documents in the archives of the KGB's First Chief Directorate, foreign intelligence. Some of the "Miktrokhin Archive" is on-line at the Woodrow Wilson International Center for Scholars.
Mitrokhin and Andrew's book The Sword and Shield states on the first page:
This book is based on unprecedented and unrestricted access to one of the world's most secret and closely guarded archives--that of the foreign intelligence arm of the KGB, the First Chief Directorate (FCD). Hitherto the present Russian foreign intelligence service, the SVR (Sluzhba Vneshnei Razvedhi), has been supremely confident that a book such as this could not be written. When the German magazine Focus reported in December 1996 that a former KGB officer had defected to Britain with "the names of hundreds of Russian spies," Tatyana Samolis, spokeswoman for the SVR, instantly ridiculed the whole story as "absolute nonsense." "Hundreds of people! That just doesn't happen!" she declared. "Any defector could get the name of one, two, perhaps three agents--but not hundreds!"
The facts, however, are far more sensational even than the story dismissed as impossible by the SVR. The KGB defector had brought with him to Britain details not of a few hundred but of thousands of Soviet agents and intelligence officers in all parts of the globe, some of them "illegals" living under deep cover abroad, disguised as foreign citizens. No one who spied for the Soviet Union at any period between the October Revolution and the eve of the Gorbachev era can now be confident that his or her secrets are still secure. When the British Secret Intelligence Service (SIS) exfiltrated the defector and his family from Russia in 1992, it also brought out six cases containing the copious notes he had taken almost daily for twelve years, before his retirement in 1984, on top secret KGB files going as far back as 1918. The contents of the cases have since been described by the American FBI as "the most complete and extensive intelligence ever received from any source." (Sword and Shield, p. 1)
...Because material from the archive was passed to so many other intelligence and security services, however, there were, unsurprisingly, some partial leaks abroad. The first, slightly garbled reference to Mitrokhin's archive occurred in the United States nine months after his defection. In August 1993 the well-known Washington investigative journalist Ronald Kessler published a bestselling book on the FBI based in part on sources inside the Bureau. Among his revelations was a brief reference to a sensational "probe by the FBI into information from a former KGB employee who had had access to KGB files":
"According to his account, the KGB had had many hundreds of Americans and possibly more than a thousand spying for them in recent years. So specific was the information that the FBI was quickly able to establish the source's credibility . . . By the summer of 1993, the FBI had mobilized agents in most major cities to pursue the cases. A top secret meeting was called at Quantico [the FBI National Academy] to plot strategy." [Kessler, The FBI, p. 433]
Kessler did not name any of the "many hundreds of Americans" identified by the defector. An unnamed "US intelligence official" interviewed by the Washington Post "confirmed that the FBI had received specific information that has led to a 'significant' ongoing investigation into past KGB activities in the United States," but declined to be drawn in on "how many people are implicated." Time reported that "sources familiar with the case" of the KGB defector had identified him as a former employee of the First Chief Directorate, but had described Kessler's figures for the number of "recent" Soviet spies in the United States as "highly exaggerated."
Mitrokhin's notes do indeed contain the names of "many hundreds" of KGB officers, agents and contacts in the United States active at various periods since the 1920s. Kessler, however, wrongly suggested that this number applied to "recent years" rather than to the whole history of Soviet espionage in the United States. Though his figures were publicly disputed, the suggestion that the KGB defector had gone to the United States rather than to Britain went unchallenged. (Sword and Shield, p. 15)
The Wikipedia entry on the Mitrokhin Archive has a very good write-up and external links and lists the books Professor Christopher Andrew and Vasili Mitrokhin worked on together:
Andrew, Christopher; Vasili Mitrokhin (1999). The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB. Basic Books. ISBN 0-465-00310-9.
Andrew, Christopher, Vasili Mitrokhin (1999) The Mitrokhin Archive: The KGB in Europe and the West. Allen Lane. ISBN 0-713-99358-8.
Andrew, Christopher; Vasili Mitrokhin (2000). The Mitrokhin Archive: The KGB in Europe and the West. Gardners Books. ISBN 0-14-028487-7.
Andrew, Christopher, Vasili Mitrokhin (2000). The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB. Basic Books. ISBN 0-4650-0312-5.
Andrew, Christopher; Vasili Mitrokhin (2005). The World Was Going Our Way: The KGB and the Battle for the Third World. Basic Books. ISBN 0-465-00311-7.
Andrew, Christopher, Vasili Mitrokhin (2005). The Mitrokin Archive II: The KGB and the World. Allen Lane. ISBN 0-713-99359-6.
Wikipedia helpfully notes:
The Questia Online Library hosts The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB. (Login required) The entire work is complete with linked footnotes and references.