MOSCOW - Published 40 years ago in Paris, Russian dissident writer Alexander Solzhenitsyn's masterpiece, "The Gulag Archipelago", revealed the shocking truth about Soviet terror and changed the way the USSR was viewed in the West.
When Solzhenitsyn's mammoth tome hit bookshops on December 28, 1973 the shock was enormous as it brought to light the horrific scale of the repression under Soviet dictator Joseph Stalin.
The word "gulag" - a Russian acronym for Main Camp Directorate - refers to the vast Stalin prison system that left millions dead, and has become synonomous in all languages with brutal detention camps.
First published by YMCA-Press, the Soviet emigrant publishing house, the book was translated into 40 languages and some 10 million copies were printed around the world.
"Judged by how much impact a book has on the course of world history, this is certainly the most influential book of the 20th century," said Solzhenitsyn's literary agent, French publisher Claude Durand.
'The sinister world of grey camps'
"Solzhenitsyn's book was a shock to us," the Nobel Peace Prize winner and Soviet dissident Andrei Sakharov, who died in 1989, wrote in his memoir.
"From the first pages arose the sinister world of grey camps surrounded by barbed wire, torture chambers... millions of our citizens vanished in glacial mines of Kolyma."
It did not take long for the Kremlin to retaliate. Two months after the book's publication, Solzhenitsyn was arrested, stripped of his citizenship and expelled from the USSR.
He returned to Russia in 1994, after 20 years in exile.
Back in the Soviet Union, Russian copies of the Paris edition had been smuggled into the country, re-typed on typewriters or photographed.
To have a copy of the book at home or lend it to a friend meant to risk being sent to prison for "spreading anti-Soviet propaganda."
In 1978 the dissident Balys Gaiauskas was sentenced to 10 years in jail for translating the book into Lithuanian.
Solzhenitsyn wrote "The Gulag Archipelago" with the help of numerous former prisoners who contacted him after the 1962 publication of his groundbreaking novella "One Day in the Life of Ivan Denisovich", the first work in the USSR that spoke about the Stalinist camps.
That book was published during the short period of de-Stalinization initiated by the Soviet leader Nikita Khrushchev.
To write it, Solzhenitsyn had used his recollections of the seven years he spent in the Gulag for criticising Stalin in a letter to a friend.
"Thousands of ex-prisoners wrote to me after the publication of 'Ivan Denisovich'. I then realised that fate sent me what I needed. I got material for 'The Archipelago' thanks to them," Solzhenitsyn said in a 2007 interview, a few months before his death.
The eyewitness testimonies are what make the book so valuable. They helped reveal the torture of prisoners, revolts and escapes through the boundless taiga as well as the deaths of prisoners from hunger and cold as they were forced to labour at temperatures of -50 degrees Celsius during the harsh Siberian winters.
Solzhenitsyn worked on the book for 10 years, mostly in the greatest secrecy as Khrushchev's de-Stalinization campaign turned out to be short-lived and the KGB secret service had him shadowed.
A network of loyalists, whom he called "invisibles", assisted him by taping, microfilming and hiding manuscripts and then smuggling them to the West where they waited for a green light for publication.
"He worked in very difficult conditions. He always had to plan in advance where to hide his manuscripts. He had been writing without ever having before him the entire body of his work," said one such "invisible" helper Elena Chukovskaya.
"He had been making notes in his notebook: to insert this in this chapter, this chapter is hidden in Estonia, another Moscow..." she said.
"The Gulag Archipelago" remains "the most important book for the people of my generation and has not lost its importance today," said Arseny Roginski, the president of the non-profit group Memorial, which studies and documents the Soviet Union's totalitarian past.
"It perfectly describes the birth and operations of the Gulag, always placing people at the centre of the story - something that only a great writer could do," said Roginski.
Until the fall of the Communist regime, Solzhenitsyn spent all the revenues from the book's sales helping Soviet political prisoners.