This is the first of the books in this CWA winners sequence that was a re-read for me. You'll no doubt know already that it's a very highly regarded book by a very highly regarded author, though opinions seem to differ about why he should be highly regarded—that is, which of his works reward sustained attention, and which are negligible. "The Spy", though, is almost always placed in the former category. A quick search will find many appreciations far more accomplished than what I'll offer.
The story centres around Alec Leamas and his role in a complicated plot hatched by the British secret services to protect their interests in East Germany. Leamas himself is somewhat in the dark about what the plot and his role in it is. For the sake of those who might be reading this before reading the book, I won't say much more about that. But it is worth noting how the story coheres with the storytelling. Part of what keeps the novel tense is that we are mostly uncertain what the plot is or what the interests being protected are. Elisions, ellipses, and doubts abound; at no point can we rely totally on what we're apparently being told about what's going on; we are repetitively forced to revise our understanding as the solid ground shifts; and yet the reader doesn't feel manipulated or cheated, and this in a novel that is entirely concerned with manipulation and cheating. That's a very difficult trick, convincing the reader to trust you while constantly betraying them.
The other notable thing about the novel is the air of absolute amorality. Le Carré was surely not the first author to present espionage as essentially a high-stakes game among equally compromised players, but the point here is that even the players seem to acknowledge this. There are some half-hearted attempts at justification of deaths and lies and accommodation of evil in pursuit of further goals, but the sense is that those goals are merely further, not higher. We go on because we have started and we don't see how to stop.
The authorial backstory inclines us to take this as an accurate account of how things really stood in early Cold War spying. Le Carré was himself a secret agent, and so seems to be a reliable storyteller in some respects, however much his technique depends on unreliability. There's much to question about how reliable he is—the things he often said about the genesis of this novel are flatly contradicted by evidence. One thing that does ring true is his claim that he wrote, or least edited, in a rush. This is because the chronology of the novel makes absolutely no sense. This should actually matter greatly for its success, given that the story really revolves around the pedantic recapitulation and confirmation of chronologies and their details. It's a mark of how good the storytelling is that it doesn't matter. (one might say the same about the fact that there's an important role for a gullible young woman whose gullibility is arguably exaggerated to a point of implausibility)
I could dig deeper into analysis of the book, but as I mentioned, others have excavated more ably, so I'll just end by saying that the minor weight of my opinions is added to that of those others: this is a properly great novel.