A chilling expose of KGB and the reach of its tentacles; KGB Today: The Hidden Hand, by John Barron. New York: Reader's Digest Press (distributed by Random House). 489 pp. $19.95.

If the Soviet Union is, as Churchill said, ''a riddle wrapped in a mystery inside an enigma,'' the wrapping surely bears the initials ''KGB'' - Russian for Komitet Gosudarstvennoy Bezopasnosti, Committee for State Security.

This is the arcane state within a state, the formidable 400,000-member police apparatus which spies on Soviet citizens, collects intelligence abroad, tries to influence Western public opinion, abets ''world revolution,'' bankrolls and staffs everything from peace campaigns to terrorist groups, and also plots assassinations.

John Barron - senior editor at Reader's Digest, a former US naval intelligence officer, and a newspaperman for most of his life - has given us a book, ''KGB Today: The Hidden Hand,'' which probes deeper into the secret Soviet agency than any other book yet written. Mr. Barron first ventured into this subject in 1974, with ''KBG: The Secret Work of Soviet Secret Agents,'' which got an enthusiastic response from critics and readers and became a best seller.

The author is passionate in his condemnation of communism, of the security force that buttresses its ''tyrannical power'' within the USSR, and of attempts to expand Soviet influence abroad. Yet in relying, as Barron does, on the testimony of several ex-KGB officers, he dispels the notion that a Reader's Digest senior editor might allow conservative biases, if they may be called that , to exaggerate what he describes as the KGB menace to freedom. Too much has been uncovered by too many other sources of information on the KGB to cause the reader concern over cold-warriorish exaggeration on the author's part.

Barron must have found that zeroing in on the agency headed by Soviet leader Yuri Andropov for 15 years to have been as difficult as tracking a nuclear particle. Yet, like a proton, the KGB does leave traces. Barron picked up and followed those traces, largely through interviews with Soviet defectors and former KGB spies such as Arkadi Shevchenko, Vladimir Andreevich Kuzichkin, and Stanislav Levchenko. The veracity of his analysis depends, to a degree, on their candor. But because of the recurrent parallels Barron found between their independent accounts, the overall authenticity of his conclusions seems more likely.

In the case of Shevchenko, Barron recounts the drama of a Soviet official's harrowing trip in out of the cold. Before his defection in 1978, Shevchenko was one of Foreign Minister Andrei Gromyko's top aides on disarmament at the United Nations. He was privy to many secrets.

Prior to his defection, Shevchenko began handing over Soviet secrets to Washington. The KGB's Moscow Center got suspicious when it noticed how well the Americans seemed to anticipate Soviet positions on arms-control issues. Finally, Shevchenko was warned by a KGB friend that Moscow was on to him: ''If you get back,'' he warned, ''you'll never get out again.'' Shevchenko chose not to return, but, as Barron shows, to provide a wealth of information to American intelligence officials and policymakers.

Another of Barron's sources, KGB Maj. Vladimir Kuzichkin, who defected to the British in Tehran in the summer of 1982, had been an attache at the Soviet consulate in Iran, where he plotted assassinations and sabotage, and trained ''illegals'' in Iran at a time when the Shah's power was ebbing.

Kuzichkin also became well informed about KGB activities in Afghanistan. Barron relates the KGB major's story of the penetration by Soviet agents of the kitchen in the presidential palace in Kabul, the attempt to poison President Hafizulla Amin's fruit juice, and, when that failed, the storming of the palace in the fall of 1979 and gunning down of its occupants. According to Barron, the Afghan coup was ordered by Andropov and the Politburo itself.

Stanislav Levchenko provided Barron with perhaps the most tantalizing material in his book - that on KGB efforts to exploit Western weakness. ''Look where your vulnerabilities are,'' Levchenko said, ''and there you will find the KGB.'' Because democracy depends on the freedom to differ, on the tolerance of the most radical political ideas, Western society produces far more opportunities for espionage and subversion than Soviet society. Moreover, because democracy depends on the broadest circulation of news and information, additional opportunities present themselves to people of dishonest intent who would spread disinformation, produce forged documents, and otherwise distort the lines of communication.

Such subversive work became the metier of Major Levchenko, who was sent to Japan in 1975 in the guise of a correspondent for the Moscow New Times. One of Levchenko's principal assignments was to penetrate Japanese political parties and become close to legislators within the Diet. Levchenko reaped a bounty of information and even succeeded in co-opting a leader of the Japanese Socialist Party.

In a chapter entitled ''The Main Enemy,'' Barron shows that the KGB and the Politburo regard the US as the foremost adversary. He also explores the ways in which the KGB conducts industrial espionage, gathering American know-how and equipment.

In his concluding chapter, ''Fighting Back,'' Barron gives his own views on what the West might do to counter the KGB threat. His countermeasures include mass expulsions of KGB personnel from Western countries; stringent enforcement of prohibitions on export of Western technology to the East; beefing up the clandestine service of the CIA, whose capabilities, he says, have eroded in recent years; and keeping the public better informed.

Most of all, Barron says, the West needs to deprive the Soviets of their presumed prerogative to ''steal, lie, cheat, subvert, intrude into the affairs of nations, foment terrorism, and incite and support wars throughout the world'' on one level while conducting diplomatic relations on another - a ''double standard,'' he insists, which the ''Soviet Union has succeeded to an astonishing degree in persuading the world to accept.''