Bought from a Sevenoaks School library sale many years ago along with several other Le Carre books; I'd never read this one for some reason.
George Smiley, retired, helps a colleague to figure out the events that have lead to the Circus's downfall. What we have, therefore, is effectively an understated whodunnit with a lot of espionage jargon thrown in — but it manages successfully to avoid ever getting boring or repetitive. It's more character-driven that the other Le Carre stories I remember, mostly because Smiley's forced to go around talking to his ex-colleagues for most of the book rather than taking action himself.
I'll have to dig out the rest of my Le Carre collection, I think. Recommended.