At his best, John le Carré's writing seems not only prescient but prophetic. His latest novel deals with extraordinary rendition of suspected terrorists but also foresees the collapse of the world financial system. Tommy Brue, the Scottish inheritor of a moribund private fund based in Hamburg, has long ago come to believe "the staple of your private banker's life is not cash, bull markets, bear markets, hedge funds or derivatives. It is cock-up." What he doesn't realise is that his late father had allowed the bank to function as a launderette for mafia funds, and is suddenly faced with a Chechen fighter who has entered the country in a shipping container and wishes to make a withdrawal. Le Carré draws a picture of weary old-school spooks supplanted by "the swiftly risen managers of the post-9/11 boom trade in intelligence and allied trades". But it's done with such surety it's impossible not to be impressed with how the great chronicler of cold war subterfuge has slipped into his new role as a profound fictional commentator on the "war on terror".

A young Muslim asylum seeker called Issa arrives in Hamburg, injured and starving. Around his neck is the key to a deposit box stuffed with the fruits of his KGB father's crooked deals with a private British bank. The banker Bruce and the mysterious Issa form a trinity with Annabel, a passionate human rights lawyer, and their activities come to involve the intelligence services of Britain, US and Germany. Because of the abridgement, you have to listen carefully, but it's worth it for le Carré's narration. His voice becomes a subtle device in this world of espionage, counterespionage and interrogation. So gentle and so measured, you're quickly caught in the web he spins.

It would be understandable were John le Carré to sit back, plump up the laurels (if you can do that to laurels) and rest up. In a writing career spanning five decades he has, after all, defined the spy novel, lifting it into the realms of literature, and given us some of the most memorable characters, set pieces and films of the post-1945 era. But he is stubbornly, exuberantly determined to keep exploring, in a world beset with wholly new paranoias, the men and (equally crucially) women who do bad and good by stealth. His new novel is basically a tale of guilty anger - on the part of the Hamburg spies who failed so miserably to latch on to Mohammed Atta and his colleagues; and on the part of the Brits and the Yanks who, desperate for success, are prepared to crawl over anyone for the sake of one small triumph, one imam they can 'turn'.

Into Hamburg, then, sneaks a tortured Russian, possibly a Chechen, with scars both mental and physical and, most pertinently, the key to a safety deposit box containing the substantial and wholly ill-gotten gains of his late and despised father, one of the KGB colonels who used Western banks to turn black money white in the dying days of Soviet Russia. Enter the likable but hapless owner of the British (but Hamburg-based) private bank that had been used for these 'Lipizzaner' deals (the famous horses are born black but turn white with age). Enter a difficult, delicately drawn female human-rights lawyer who sees in Issa, the refugee, the chance to make amends for previous deportations she failed to prevent. Enter, sotto voce, at least three national espionage networks, watching and planning their three-dimensional chess. The Germans, led by the intensely affable Gunther Bachmann, the book's finest character, see a chance to use Issa to compromise a 'moderate' Muslim TV cleric whose charities follow some odd conduits. The Americans want to come in all guns blazing, not just figuratively. The Brits want to skulk, threaten, wheedle, double-cross and steal credit.

What le Carré has always done terrifically is to capture the nuances of the spying game. His spooks are wonderful. You find yourself believing you are in that room, quietly rooting for whoever commands your allegiance at that moment. He paints the scene so fully in his own mind before writing that you forget you're reading fiction: every cough, every glance, each sip of bottled water feels as if it were part of a scrupulously honest documentary. It is also a delight to read a man who believes in proper continuity, when so many lesser thriller writers have waiters arriving with the first course three seconds after the diners have met.

Where le Carré falls down, I think, is in capturing the burgeoning (or is it?) love triangle between the pretty lawyer, the rich but rubbish banker and the (frankly unlikable) refugee. Did Issa boff Annabel? Will Tommy get her instead? Frankly, who cares? This too-huge subplot fails to grip, and simply points up how much more riveting the real action is. Le Carré's minor characters are never less than spot-on, but his three main ones are oddly shoehorned into emotions that we, the readers, fail to share with them. (And besides, Issa is so annoying that if the gung-ho Americans ever did end up fitting him for a dinky orange boiler-suit, I don't think too many readers would be weeping.)

But these failures aren't too disastrous. Relish, instead, the knowledge this book imparts about the men who have learnt to talk just below the level of hotel music, and say small things with huge import; about the impossible moral Möbius strip handed to Western liberals by Islamicist jihad. In A Most Wanted Man you are, unlike the modern world, in thrillingly deft, safe hands.