Asked to choose between love and friendship, most people in western cultures would immediately, nervously, tick love. It makes the world go round; it has a beginning, a middle and an end happy-ever-after; its sincere expression, according to Stendhal, always possesses a character of beauty. That's the faith in which we were all brought up. But even in our own times, there have been societies which ticked the other box.

My most vivid memory of the old Soviet Union, or  if you like  of Russia in hard times, was about institutionalised friendship. In Leningrad, I came to know circles of friends, mostly women, who met every week over cake, tea and wine to exchange news, thoughts and plans. But these gatherings were far more than tea parties.

They were ceremonies of mutual support, of a serious quality of friendship which demanded utter loyalty. If parents disappeared overnight, they could be certain  as the jail doors slammed on them  that friends from the circle would rescue their children left alone in the apartment and bring them up as their own.

Friendship, then, was the primary relationship. It was reliable, and it was for life. Love was marvellous, irreplaceable, but not to be counted on. Men, especially, came and went; that was how the world was and all the more so in times of war or state terror. Husbands and lovers appeared at those circle sessions, often with a bottle of cognac or a carton of Kent cigarettes, but did not always reappear.

That is what makes this book remarkable. There are many Russian friendships in it, maintained and tended across continental distances, across barbed wire and the lapse of years. But at the centre is a love story or perhaps, more accurately, a story of love.

It's that story, as expressed in the lovers' own letters, which readers of the book should hold on to. Unhappily, a Figes publication now comes with a health warning. Uproar over pseudonymous reviews of his own work has been followed by the current row over alleged errors and distortions in The Whisperers, his previous book about Stalinist tyranny. Irina Ostrovskaya is chief researcher at Memorial, the institution set up to record Soviet crimes and the fate of some 24 million people who passed through the Gulag camps or forced exile. A fierce critic of The Whisperers, Ostrovskaya now denounces Just Send Me Word as "melodrama  I found a lot of things with which I did not agree". It's a fair guess that there will be challenges about fact and detail in Figes's narrative here. But that must not detract from the marvel of the letters themselves, the superb faith that can exist between two unimportant, "superfluous" human beings.

Lev Mishchenko and Svetlana Ivanova did not hurl themselves into one another's arms. Lev said, when he was an old man: "It was not that we fell madly in love with one another, but there was a deep and permanent affinity." Svetlana  Sveta  said in her own old age: '"I knew he was my future from the start." But it wasn't for several years, long after they knew that their lives were bound together, that she wrote in a letter that "I want to tell you just three words  two of them are pronouns and the third is a verb (to be read in all the tenses simultaneously: past, present and future."

The endurance of their love, their refusal to give one another up in the face of everything the Soviet 20th century could thrust between them, was a matter of obstinate, non-fanatical faith. When they met as Moscow university students before the second world war, they were the same age but not at all the same in background. Lev was an orphan whose parents had been shot as "bourgeois counter-revolutionaries" by Bolshevik troops; he was brought up by loving aunts and a grandmother. Sveta's family were "technical intelligentsia", her father an industrial scientist who had quietly left the Communist party in disillusion. Their courtship was intense and chaste: long walks, reading poetry together (Akhmatova and Blok), cycle rides to the countryside. Three years went by. Then the war came.

Lev joined up, and was captured in the first victorious onrush of the German armies. He could speak German, and in the prison camps he was pressured to change sides and join the anti-communist "Vlasov" army. He refused, but occasionally used his German to translate camp orders. For that, Soviet security arrested him after the war as a "fascist collaborator" and sentenced him to death, commuted to 10 years in a labour camp at Pechora in the far north. He had been posted missing in 1941.

Sveta feared that he was dead, but steadfastly refused to give up hope  in the spirit of Simonov's much-loved wartime poem "Wait for me " It was not until July 1946 that he managed to tell her where he was. At once she wrote to him, the first of 1,246 letters exchanged between Sveta in Moscow and Lev in the gulag over eight and a half years. Miraculously, all have survived. Thousands of handwritten pages carefully numbered, they form one of the most precious acquisitions of Memorial, the institute and archive established to preserve the memory of the gulag and the crimes of soviet communism.

At first Lev was set to dragging logs to the "wood combine", crushing labour which he would probably not have survived for long. But his life was saved when he was recruited by Georgii Strelkov, an ardent Old Bolshevik serving 25 years for a fictional offence, who was running the combine's research laboratory. Lev found himself in a group of likeable, intelligent men who kept themselves minimally warm and fed. And in Strelkov's "salon" it was possible to write forbidden letters, posted by "free" workers living near the camp.

Sveta hadn't seen Lev for five years. Her first letter, after she discovered he was alive, starts with wonderfully Russian asperity: "Levi, if I didn't know that actions should be judged by their motives and not by their results, I would reproach you for your silence " But her feelings had not been changed by war and tyranny. "I'm becoming stubborn, Lev. How many times have I wanted to nestle in your arms but could only turn to the empty wall in front of me  we will get though this, Lev."

She was working in a tyre research unit, living with her parents in Moscow. Her letters avoid "sentimental words about love (both lofty and cheap)", but give a unique sense of her Moscow life, the journeys to work, the clothes she wears ("my summer grey-green coat is still alive") the curious people she meets on trains, and her research ("A Project on the All-Union State Standard for the Determination of Frost Resistance using the Impact Fracture Method").

Lev's letters are more passionate, and his moods more uneven. He felt blessed and saved by Sveta's love, but was visited by doubts about his own worthiness. She spent much energy pumping optimism into him, as the years passed. She and her family sent parcels to him and to his friends at Pechora: warm clothing, food, soap and toothpaste, pens and ink, "glucose and ascorbic acid (Vitamin C to the unenlightened)  eat it, for God's sake". But "Vitamin C" soon became part of a code between them. It meant bribing the guards with vodka, while "Vitamin D" was bribes of money. "Umbrella" and other words about rain meant the gulag itself.

Sveta decided that she must visit Lev. This was utterly illegal; discovery would have wrecked her life and that of her family. It was a forbidding journey of almost 4,500km; it required deception, bribery, patience, courage and cunning. But after a year of joint plotting, she managed to blag her way across northern Russia, through cordons and past sentries, and spend a night with Lev in an empty room. She did it again, and then again. Sometimes they could only be together for a couple of hours, holding hands in a crowded office. Sometimes more than a year could pass without a meeting, so that they saw each other only in dreams. But it was worth it. As Sveta said, they were stubborn.

Orlando Figes, selecting and then interpreting this mass of letters, makes them tell two kinds of story. The first is a uniquely detailed narrative of the gulag, of the callous, slatternly universe which consumed millions of lives and yet, through its vastness, developed chinks in which lucky prisoners might construct a shaky independence. The second is about two people determined not to lose each other. Lev suffered most; the moral degradation he saw in the camps at times made him lose hope. He tried to keep that from Sveta, who was more upset by his depression than by the wasted years of separation. About time and age, they were fearless. Lev wrote: "You once said, and quite correctly (you were sitting at your table with 'Principles' or 'Thermodynamics', I've forgotten exactly but I remember it was evening and a table light was buzzing, and I was standing near the piano) that without changing over time people would not become themselves "

Lev and Sveta had no time for the ideology; they hated the injustice and cruelty of the system. But Lev took pride in Soviet achievements such as the Volga-Don canal, even though he knew it had been dug by the labour of slaves like himself. Sveta, although she despised the bureaucracy and the jargon, was a party member and in some ways a model of the busy "Soviet woman". They were not "dissidents", just two people who put their love first.

A terrible suspense builds up in this book, as Lev's sentence nears its end. They both take dreadful risks. But it would be wrong to give the end of their story away. Enough to quote Simonov again:

"Only you will know how I survived;
It was because you waited."

 Neal Ascherson's Black Sea is published by Vintage.

 A response by Orlando Figes can be read here

 This footnote was added on 20 November 2012. A paragraph about a controversy relating to The Whisperers, an earlier work by Orlando Figes, that should have been the sixth paragraph of the review was left out due to an editorial error. It was restored in July.

"Death solves all problems," declared Joseph Stalin, adding with his customary brutality: "No man, no problem." Anyone found guilty of threatening the Soviet state by his deeds or even thoughts would be eliminated. During the Great Terror of 1937-8 close to two million Russian army chiefs, priests, professors and other "enemies of the people" were shot or sent to the Gulag. The executions were carried out by many willing henchmen but Stalin was the dominant will behind them. With cynical adroitness, he set families against families; the children of Gulag internees were shut up in reformatories and often not heard of again. Their blighted personal lives ill-conformed to the Stalinist ideal of a happy childhood.

For all his cruelty, Stalin was not delusional. He knew very well that "confessions" extorted from his so-called enemies were only rarely true. (One victim, Stalin liked to joke, had been persuaded under torture that he was the author of Eugene Onegin.) Unlike Hitler, Stalin did not see enemies everywhere: rather, he invented enemies because he needed them. Underlying his strategy of annihilation and persecution was the deeply rooted principle (inherited from Lenin) that enemies were more useful to Soviet power than friends. Stalin's power was in fact directly proportional to the extent of the perceived enemy threat, whether it was kulaki peasant proprietors, American spies or, later, Jewish doctors.

It was not difficult for Stalin to seek out his enemies. By the dawn of the 1950s some 10 million spies were operating in the Soviet Union. Information was gathered on so-called "wreckers" in every walk of life  name, address, profession  and set down with lapidary coldness in official communiqués. Fearing guilt by association, friends distanced themselves from each other. The Bolshevik virtue of tverdost  hardness  demanded an indifference to the trap door disappearance of those you loved.

In Just Send Me Word Orlando Figes tells the true story of two young Moscow scientists, Svetlana Ivanova and Lev Mishchenko, whose love for each other endured the Gulag and Stalin's attempts to "reforge" them into dutiful Soviet citizens. Neither Svetlana nor Lev was at heart anti-Soviet; they believed in the forward march of the "new Soviet man" through technology, and at first they revered Stalin as the great vozhd (leader) of the Russian people. Fiercely motivated and bright, Svetlana saw in Soviet communism an intellectual calling and hope for the future.

All that changed in June 1941 when Hitler invaded the Soviet Union. The carnage as German troops advanced across the Stalin line was appalling: roughly 27 million Soviet citizens were to perish in the conflict they called the great patriotic war. Lev hurried to sign up in defence of his country, and showed great courage as a Red Army officer in the forests near Smolensk. He was captured by German troops, however, and interned in Buchenwald, where the guards beat the prisoners with a bestial insouciance (using rubber truncheons that left no mark on the skin).

At the war's end, in spite of all he had suffered, Lev was interned in the Gulag as a suspect German spy. It made no difference to Stalin's interrogators that he was innocent. Convicted of "treason to the motherland", he was given 10 years in the corrective labour camp of Pechora in the far north of Russia. Other enemies were "purged" (a euphemism for judicial murder which the Soviet dictatorship has added to the language) or else sent to die in the frozen immensity of Siberia.

Against all the odds, Lev managed to smuggle letters out of the Gulag to Svetlana back in Moscow. He did not think of the possible repercussions for her in receiving correspondence from a convicted "enemy of the people": he lived for "Svet" and ignored all risks. As Lev persisted with his secret correspondence he began to receive post in return from Svetlana. The letters gave a slender meaning to his life and a sense of a connection to the world he had lost. He concealed them beneath floorboards in his barrack.

Fortunately, the correspondence is preserved in its entirety in a Moscow archive. Running to some 1,246 letters, it is the biggest known private collection relating to the Gulag. From it, Figes is able to build a picture of life in Pechora with its watch towers and isolation cells, where violence was the language understood by all.

Lev is initially shocked by the "criminality" shown by his fellow inmates. The prime stimulus in the camp was to find food; the food either had to be stolen or bought from the camp's gangster elite. Very few prisoners were able to remain faithful to the "decencies" they had left behind in the free world. "You really do become a savage and malicious animal," Lev wrote to Svetlana, adding that he would have been appalled by the thieving as a free man (in the Gulag, thieving was considered a matter of pride).

The guards were often drunk and violent, yet the violence was administered less out of hatred than out of routine, Lev suggests  the way a man might beat his dog. The arbitrariness and cruelty of tsarist authority had found its grotesque mirror image in the labour camp: day to day, prisoners had no idea what would happen to them. When Svetlana visits from Moscow one day, having made the 900-mile rail journey to Pechora without official permission, Lev can scarcely contain his excitement. "My Sveta, how wonderful it is that there is you I see only you."

On his release in 1954, however, one year after Stalin's death, Lev appears to be disorientated and mildly depressed. After the nightmare intensity of the Gulag, everything seemed colourless, futile and false to him. "He felt none of the euphoria he might have expected," Figes comments. "In some ways he was sad to be leaving."

Lev had grown up dreadfully during his Gulag captivity. His innocence and much of his emotional life had been ripped out of him. Yet, in some awful way, the Gulag was his university and the place where he learned the ways of the world.

He was overjoyed to be reunited with Svetlana but, like all exiles, he felt that he had come back to a different world that had moved on without him. Everywhere he went in Moscow, employers were suspicious of his "criminal record". There was no glory in having been a Gulag internee; the shame was indelible.

But in 1956 his fortunes changed when Khrushchev denounced Stalin and his "cult of personality". The unmasking of Stalin as a dictator was a turning point for the 20th century; with it, the die was cast for Gorbachev's glasnost in the mid-1980s.

Having married Svetlana, Lev was finally able to return to his pre-war work at Moscow University's Institute of Nuclear Physics, where he stayed until retirement. When, in 2008, Figes visited Lev and Svetlana in Moscow, he found a contentedly married couple who had come through hard times together. Just Send Me Word, grimly absorbing, conveys the pity of the Stalinist Gulag with integrity and proper sympathy.

Ian Thomson's biography of Primo Levi is published by Vintage.