A Legacy of Spies (George Smiley #9) by John le Carré

Peter Guillam, staunch colleague and disciple of George Smiley of the British Secret Service, otherwise known as the Circus, is living out his old age on the family farmstead on the south coast of Brittany when a letter from his old Service summons him to London. The reason? His Cold War past has come back to claim him.

Intelligence operations that were once the toast of secret London, and involved such characters as Alec Leamas, Jim Prideaux, George Smiley and Peter Guillam himself, are to be scrutinized by a generation with no memory of the Cold War and no patience with its justifications.

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.)