The undisputed master of espionage fiction returns with a riveting book--his first Smiley novel in over twenty-five years.      The Cold War years are history. Peter Guillam, once the effervescent colleague and confidant of George Smiley, head of the British Secret Service's Covert Operations, has retired to manage the family farmstead in remote Brittany. So why does his former Service, once called the Circus, summarily demand his presence in its hideous new bastion beside th