John le Carré’s triumph (and consequent burden) is that he created characters and language so evocative of the spy world that they became more real in readers’ minds than real people or events. This happens occasionally with books or movies: Our images of the old South are inseparable from the way it was portrayed in Gone With the Wind. It’s said that even real-life members of the Mafia learn how mobsters are supposed to talk by watching The Godfather.

So, too, with le Carré’s books. Intelligence officers nowadays speak of “moles,” the word le Carré popularized for what used to be known as “penetration agents” or “sleepers.” Every reader knows the basics of surveillance tradecraft, thanks to le Carré’s evocation of the “pavement artists” who work for the “Lamplighters” division of the “Circus.” And George Smiley is surely more vivid than any actual senior officer of MI6 ever was. Pity the real-life “C,” who had to compete with Alec Guinness’s portrayal of Smiley in the BBC serializations of le Carré’s books.

The challenge for le Carré is that these celebrated characters now belong as much to readers as to the author. When he writes in his new book that Smiley is “owlish,” some readers may roll their eyes—please, we know what Smiley looks like. I suspect that’s one reason le Carré, after writing Smiley’s People, mostly walked away from his original characters and created new ones for the second half of his career. Many of le Carré’s later books weren’t as good, in my judgment, but he had some unencumbered space in which to write.

Le Carré’s challenge is the sort that other writers dream about. Three of his books defined the spy-fiction genre: The Spy Who Came in From the Cold (1963), Tinker, Tailor, Soldier, Spy (1974), and Smiley’s People (1979). (I’m leaving out The Honourable Schoolboy of 1977, which I think is a lesser work.) How do you improve on perfection?

Le Carré has found a clever solution to this problem in A Legacy of Spies. He has written a kind of prequel to the book that made him famous, The Spy Who Came in From the Cold. And he has chosen as his protagonist not George Smiley (who is a Yoda-like presence offstage for most of the book) but his chief lieutenant, Peter Guillam. Fans will recall that Guillam was Smiley’s most trusted colleague, the man who helped Smiley recruit Alec Leamas for a diabolical operation across the Berlin Wall; then root out the treacherous Bill Haydon, the mole who had penetrated the Circus; and finally trap the cunning Karla, Smiley’s arch-nemesis at Moscow Centre.

But who was Guillam? We remember from earlier books that he was a hard man, a field agent who headed the “Scalphunters” unit, which carried out especially violent or dangerous operations. Like Toby Esterhase, another beloved secondary character, he had suspect foreign blood: Guillam was half-French. He was a womanizer and a drinker, too, but he left the brooding and the guilt to his mentor, Smiley. Now, in A Legacy of Spies, we see the inner Scalphunter.

As the book opens, an aging Guillam’s retirement in Brittany is interrupted by a missive from his former colleagues at the Circus: “A matter in which you appear to have played a significant role some years back has unexpectedly raised its head.” His presence is urgently required in London. The “matter,” rooted in the plot machinations of The Spy Who Came in From the Cold, is believable in this litigious, rearview-mirror age. The children of Alec Leamas and Liz Gold, who were used by Smiley in his long-ago operation to protect the Circus’s mole in East Germany, are seeking vengeance (or, if you prefer, justice). What reader can forget Gold’s death as she tried to climb the Berlin Wall at the end of the book, and the despairing Leamas’s decision to die with her? That last scene, two trusting people crushed in the service of a higher cause, set the gray tone of moral ambiguity that colored le Carré’s subsequent work.

The deaths of Leamas and Gold may have been a Cold War tragedy. But to the younger generation, it’s a potential lawsuit. A pastiche of meddlesome characters with names like Bunny and Tabitha begin quizzing Guillam about wrongful deaths caused by lawless spies, and demanding that secret records be unearthed and exhibited in court.

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This current-day legal jeopardy is the engine that drives the excavation of the past. The device is not entirely satisfying. The genius of le Carré’s early novels lies in plots that move forward as inexorably as slow-rolling waves. Overreliance on flashbacks is never a satisfying way to tell a story, especially when, as here, many of them take the form of snippets of documents and tape recordings supposedly composed long ago. Punctuating Guillam’s recollection, they interrupt the momentum of the story, and the voices of the characters begin to blur a bit.

But this is a modest criticism. I read most of the book in one pleasurable sitting. Le Carré is such a gifted storyteller that he interlaces the cards in his deck so they fit not simply with this book, but with the earlier ones as well. Devotees who want to understand the arcana of Hans-Dieter Mundt’s relationship with the Circus, or the blown network of Dr. Karl Riemeck in East Berlin, or the schemes of Inspector Mendel of the Special Branch, won’t be disappointed.

ALegacy of Spies may be the capstone to the Smiley novels. I hope not. I’d love to see this addictive author, even at 85, explore other prequels and sequels. But if it’s a summing-up, two points are noteworthy. First, le Carré seems to me to be rooting for the spies in this book—yes, with ambiguities and meditations on whether the mission was really worth it. Yet this time around, they’re unquestionably the good guys. The younger folks who are chasing them down are score-settlers whose cynicism about life is unearned.

“You’re all sick. All you spies,” screams one of the would-be avengers. “You’re not the cure, you’re the fucking disease … You live in the fucking dark because you can’t handle the fucking daylight.” Not a very convincing speech, profanity aside, and then the avenger loses his nerve to boot, and crumples before Guillam.

The quality of le Carré’s writing is a kind of tip-off as to where his sympathies lie. He is at his worst describing characters he doesn’t like (in his recent books, many of the heavy-handed, unconvincing figures are American intelligence officers), and these litigious upstarts don’t quite ring true. Meanwhile, le Carré has doubled down on the old boys: Guillam is entirely admirable, even as he lies to shield the past.

Guillam graduates at the end of the book to a Smiley-esque, world-weary angst. “How much of our human feeling can we dispense with in the name of freedom, would you say, before we cease to feel either human or free?” he muses on his way to a final encounter with Smiley. This idea, that spies under every flag have been degraded by their profession, is a signature le Carré sentiment, but in Guillam’s mouth it sounds forced. Up to this point, he has seemed a creature more of the Richard Helms “Let’s get on with it” school of espionage. The martyred Alec Leamas, as we rediscover him in these pages, isn’t burdened by such big thoughts. We learn more about the dogged loyalty that drove him to sacrifice himself on the Cold War altar.

The tough field agents like Leamas and Guillam achieve a nobility in this book that eclipses even that of our oft-betrayed hero, the reflective, German-monograph-reading Smiley. These days we clap for servicemen and women at airports and athletic events, regardless of how we feel about the wars in which they fought, and there’s a touch of that deference in A Legacy of Spies. The world of the on-the-ground operators may be gray, but we pay tribute to their steadfastness and valor.

Finally, what does this book tell us about George Smiley? For most of the novel, he is Deus absconditus. People keep asking for him, but he’s … not available. You think maybe the old boy has died or, like so many former MI6 officers, gone to work consulting for investment bankers. But no. I don’t think I’m giving anything away when I say that at the close, we encounter Smiley in a library near the Swiss–German border, reading obscure books just as he used to on Bywater Street.

There’s a wink of the eye from le Carré in choosing this setting: He told us in his fine memoir, The Pigeon Tunnel, that the Swiss Alps are his favorite place. He built a little chalet there with the money he earned from The Spy Who Came in From the Cold, and the terrain was also special for his Oxford mentor, Vivian Green, “who gave me by his example the inner life of George Smiley.”

Smiley offers some bland comments at the very end of the novel about the meaning of his long battle into the Cold War twilight, but the reader half-suspects that it’s all hooey—that Smiley doesn’t really know why he did what he did. He wouldn’t tell us in any case, nor would he consider breaking faith with the people, like Peter Guillam, who served under his command. Less is more with Smiley, as with any great enigmatic character, and I am glad le Carré has left the spaces between the words for us all to fill in.