I knew there was a Smiley book between The Spy who came in from the Cold and Tinker, Tailor, Soldier, Spy, but I had in my mind that it was A Small Town in Germany. A little research on the Internet revealed my error, although to be honest Smiley is a peripheral character in this tale, which is as much about internal politics and bureaucracy as it is about the Cold War.
The "Department" - of which we know little other than it deals with military intelligence, and is most definitely separate from the Circus - has unconfirmed reports, in the mid 1960s, of military equipment arrivals and closures in an area of East Germany. The story begins with the pick-up of a film at an airport in (neutral) Finland that goes wrong. It's obvious that the Department, which hasn't mounted any significant operations since the Second World War, is way out of its depth. But the Department, for reasons that are never made clear, does not want to deal with the Circus. The presentational reason is that the Circus doesn't deal with military material, but most obvious interpretation is that it fears being subsumed in the machinery of its larger rival. We learn most of this through one of its junior officers, Avery, who doesn't know any more about the past than we do.
Unfortunately, to mount the operation requires help from the Circus, but the misleading cover story that it is a training exercise is taken at face value, and obsolete equipment and procedures are arranged. Leclerc, in charge of the Department, and Control, at the Circus, seem to be waging a war of their own, in that inevitably polite and overtly co-operative way. Somewhat characteristically, Smiley is on hand to pick up what can be salvaged when it is all over. One is left to imagine the fate of the agent abandoned in the East, and also of the Department, whose assimilation into the larger community seems inevitable.