That's the one where Smiley the retired and discredited spy (the one who suspects the flashy/trashy new spymasters) is secretly brought back to track down a mole in the highest reaches of the MI-6 (which he calls "the Circus"- it's really "The Service"-- Her Majesty's Secret Service).
This is supposedly loosely based on the Kim Philby treasons (Le Carre was a spy there around that time), but also might be an oblique reference to the constant rumors about the CIA's upper echelon in the 60s, especially James Jesus Angleton, who himself was supposed to by "the molehunter."
There are 4 novels in the sequence, but TTSS is the most important one. It's very moody and dark about the future of Britain, and rather scathing about the "cousins" (the US and CIA) which are presented as successful opportunists.
The "Soviet Union" is the enemy, but it doesn't seem that much different from Putin's Russia. Interesting, that the "Russian" aspect ended up so much more important than "communism,' which fell away and still left that dark KGB influence.
https://en.wikipedia.org/wiki/George_Smiley
There is also a very good TV mini-series with Alec Guinness as George Smiley. It's one of his greatest roles-- fits him like a glove. There's a 2011 film (on Netflix) with Gary Oldman, which is good, but too compressed for the complicated plot. The TV series (7 episodes) is better.
= new reply since forum marked as read