"What do you think spies are: priests, saints, martyrs? They're a squalid procession of vain fools, traitors, too, yes; pansies, sadists and drunkards, people who play cowboys and Indians to brighten their rotten lives." The person responsible for this bitter rant is Alec Leamas, the deadpan fiftysomething protagonist of John le Carré's 1963 novel The Spy Who Came In From the Cold. We will refer to it as The Spy from now on, for brevity's sake, but it's worth starting any current assessment of the novel with something of a thought-experiment. The Spy is set in the early 1960s before the assassination of John F Kennedy, before the real advent of hippies, the pill, the Vietnam war, the "swinging sixties" and all the familiar counter-cultural baggage that goes with it. Its tone, if anything, is dourly 1950s, its colours grey, its weather depressing. It's worth remembering that rationing in Britain finally ended in 1954; that the second world war was a fresh memory (Leamas is a veteran); indeed, that anyone in their 70s would be a survivor of the 1914–18 war, the first world war. The action of the novel takes place half a century ago. It belongs to an entirely different world from the one we know today.

And yet, and perhaps this is the first remarkable comment to make about The Spy, its cynicism is resolutely de nos jours. One forgets just how unsparing the book is, how the picture it paints of human motivations, human duplicities, human frailty seems presciently aware of all that we have learned and unlearned in the intervening decades. The world was, on the surface, a more innocent, more straightforward place in the early 1960s: there were good guys and bad guys and they were easy to spot. One of the shock effects of reading The Spy when it was published must have been the near-nihilism of its message. It is unremittingly dark – or almost so – and this fact, I believe, lies at the root of its greatness.

The Spy is the story, to put it very simply, of a complicated act of deadly triple-bluff perpetrated by the British Secret Service against its enemies in the German Democratic Republic, as communist East Germany was then known. At its centre is Alec Leamas, sent, he believes, on a clever under-cover mission of revenge but in fact the unwitting tool of even cleverer British brains with other motives. So much so relatively straightforward, but one of the sheer pleasures of the grade one espionage novel is in unravelling its multifarious complexities and Le Carré handles the unspooling web of narrative and motive with exemplary poise.

The second remarkable aspect of The Spy is the skill with which it is constructed and written. It was Le Carré's third novel (after the highly creditable Call for the Dead and A Murder of Quality) but there is a clear sense in The Spy of a writer hitting his stride with resolute confidence. Unusually for a spy novel, Le Carré's narrative point of view is omniscient – a dangerous choice, because with authorial omniscience you cannot have your cake and eat it. If you are saying to the reader that you can enter the thoughts of any character and can comment on the action or events in your own voice, then any deliberate withholding of information counts as a black mark. The narrative house-of-cards begins to collapse; the reader's trust in the author's control dissipates immediately. Technically, on a purely writerly analysis, Le Carré seems to me to be operating at the highest levels.

There is never a sense that we are being overly manipulated – the choice of those characters whose inner thoughts he shares with us seems entirely apt – we never feel we are being narratively duped. Also, for a relatively short novel a tremendous amount is included. The ellipsis between chapter two and three is a model of how a simple change of point of view can eliminate pages and pages of laborious exposition.

Leamas's staged three months in prison covers three pages – and yet we emerge from them with a complete sense of what he must have gone through: the tedium, the loss of self-respect, the numbing brutalisation. The old adage of pouring a quart into a pint pot was never more successfully demonstrated. On a line-by-line level, furthermore, the prose is limpidly succinct and evocative. For example: "A girl performed a striptease, a young drab girl with a dark bruise on her thigh. She had that pitiful spindly nakedness which is embarrassing because it is not erotic; because it is artless and undesiring." Or: "The airport reminded Leamas of the war . . . Everywhere that air of conspiracy which generates amongst people who have been up since dawn – of superiority almost, derived from the common experience of having seen the night disappear and the morning come."

There is a real confidence exhibited here, a sense that the author knows absolutely what he is talking about. Of course, we are now aware that John le Carré (aka David Cornwell), was highly familiar with the secret world of espionage and counter-espionage but again, in 1963, this assuredness would have come across like a blast of keen cold air. The spy novel was being reshaped with The Spy – it was a paradigm shift in the genre – it would never be the same again and indeed its wider influence in literary fiction was manifold. However, as with a lot of artistic revolutions, this realisation comes with the benefit of hindsight.

I must have first read The Spy in the early 70s, I suppose, and have read it three or four times since. And I think what I relish about it – and this is maybe how Le Carré transformed the genre – is the implicit respect that he gives the reader. It is a very exciting read but it's also highly complicated. There is a lot of challenging subtext, a lot is implicit, a lot seems initially confusing. In other words, it's very sophisticated and one of the appeals of sophistication in art is the understanding that such precision, such tastes, such values, such understatements are shared. Le Carré's novel says, as it were, I know this appears unduly complex and obfuscated but you, the reader, are an intelligent person: you will follow this – you will understand what is going on, I don't need to spell it out or join the dots. The sheer aesthetic pleasure of reading is massively enhanced, thereby.

But I don't think this fully explains why I have reread the novel over the years. Novels you reread have a different role in your personal pantheon than novels you simply admire or revere. There is something troubling about The Spy that draws you back again and again. Partly it is the sense that you may have missed something – that you haven't fully unravelled the intricacies and nuances of the book. One of the aspects of the novel that always bothered me was the end. Leamas, to put it in vague terms – not to give the story away – finally realises how he has been used by his own side, how he has been fooled, manipulated and misinformed to bring about a conclusion that was the opposite of the one he thought he was colluding in. He is offered the chance to flee, to escape and climb over the Wall with the young girl he sort-of loves back to West Berlin. He and the girl are driven to a "safe" area of the Wall in a car provided for him by a double agent. Operationally and procedurally this seemed to me a huge error. My feeling was that an agent of Leamas's vast experience and worldliness would surely be aware that such a means of escape was riven with jeopardy. Yet he goes along with it and pays the price.

What had I missed? Reading the book again I now think I understand – but it does require close attention (new readers look away now). Leamas, betrayed, hoodwinked, terminally fatigued, is in a state of existential despair at the end of the novel. The opportunity to escape means nothing to him – but it does mean everything to him that the girl he is with, Liz Gold, innocent, unwittingly drawn into the Circus's plotting – should escape. Leamas knows unequivocally at the end of the book that he is going to be betrayed again (there is a crucial, easily missed, detail about a car leaving when it is not meant to) but he tries all the same to thwart that betrayal. If only he can get Liz back to the west – that is all that matters to him – he's indifferent to his own fate. So he tries to get Liz over the Wall. My reading of the last page of the book is that the British Secret Service (who have used Liz as brutally and pitilessly as they have used their trusted agent Leamas) always intended that Leamas should escape – should come in from the cold – and that Liz should die on the Wall. She knows too much: free in the west, she would be too much of a liability. A disaster, in espionage terms. She is duly shot as she tries to climb over – but Leamas still has the opportunity to make it to freedom.

George Smiley, off-screen mastermind of this devilish brew of bluff and counter-bluff, is waiting for him. Leamas hears Smiley shout: "The girl, where's the girl?" But what Smiley wants to know is not whether the girl is safe but whether the girl is dead. That is the key implication (or so I read it) – that she's never coming over and was never meant to. Leamas suddenly understands this – it is the final betrayal he suffers – and he climbs back down to the east and meets his death.

Two factors hinge on this bleak interpretation, both requiring that the reader remember clues planted early in the novel. First, the very last sentence of the book must seem baffling to the vast majority of readers: "As he fell, Leamas saw a small car smashed between great lorries, and children waving cheerfully through the window." This sentence recalls a moment of reflection some 140 pages earlier in the novel. Leamas sees it as a kind of epiphany, a revelation, that occurred while he was on an earlier mission and was driving too fast on an autobahn and almost collided with a small car with four children in the back. The near accident traumatises him and the comment is made that "men condemned to death are subject to sudden moments of elation; as if, like moths in the fire, their destruction were coincidental with attainment." In the very last sentence of the novel Le Carré directs us back to these few paragraphs in order to provide the necessary full catharsis.

Second, the concept of a "spy who comes in from the cold" seems to be fairly easily understandable – namely that the years of a spy's double life are over, that there is an end to the unceasing watchfulness, the interminable daily duplicity: he (or she) can come home. Le Carré uses the phrase in this sense in the novel but also supplies us with another reading of it very early on in the narrative, putting the words in the mouth of "Control", the head of the Secret Service, as he briefs Leamas on his mission. "We have to live without sympathy," Control muses. Then adds: "That's impossible, of course. We act it to one another, all this hardness; but we aren't like that really. I mean . . . one can't be out in the cold all the time; one has to come in from the cold . . . d'you see what I mean?"

So, "coming in from the cold" also means displaying a fundamental human empathy, of living with sympathy for others. It means the very opposite of being "hard". The paradox at the end of this superb, tough, highly sophisticated novel is that Leamas, in refusing to come in from the cold as a spy, does in fact come in from the cold as a person. His destruction is coincidental with his attainment. In his deliberate orchestration of his death he shows that he is a human being.