This is probably the finest of Le Carre's novels. His great creation, George Smiley, is repsonsible for finding a Soviet mole in the heirarchy of British Intelligence which has done immeasureable damage for decades. George is the most unlikely hero - ponderous, old, shy, retiring, but posessed of enormous compassion and iron will. This who-dunnit story plays against a general background of betrayal - the betrayal of the mole against the British state, the betrayal of the agents run by the mole, the betrayal of Smiley's wife's infidelity, the general betrayal of idealism in the Circus to the mundane self-serving ends of its leaders.
And then there is the setting - Britain in all its drab, mundane 1960's/70's glory. Drab colours, poor food, rain soaked days, steamed up car windows, snobbery and poverty. And the dialogue is second to none. So world weary, so wise. And the intelligence world rings true in this book too, it feels realistic, it feels about right. The moral ambiguity is embraced by Le Carre. Though there are heroes and villians in this book, the boundaries are fairly blurred.