Tinker, Tailor, Soldier, Spy (George Smiley #5 / The Karla Trilogy #1)by John le Carré

A modern classic in which John le Carré expertly creates a total vision of a secret world, Tinker, Tailor, Soldier, Spy begins George Smiley's chess match of wills and wits with Karla, his Soviet counterpart.
It is now beyond a doubt that a mole, implanted decades ago by Moscow Centre, has burrowed his way into the highest echelons of British Intelligence.

His treachery has already blown some of its most vital operations and its best networks. It is clear that the double agent is one of its own kind. But which one? George Smiley is assigned to identify him. And once identified, the traitor must be destroyed.

Novels by John le Carré in which the character George Smiley appears. With the exception of The Karla Trilogy, these are standalone works and can be read in any order. (In four out of nine of them, Smiley is only a minor character.)