I finally understand! 4 stars.
The blurb: 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.
If you read my recent review of The Night Manager, you might remember that having seen the recent BBC TV adaptation ruined my experience of the book. Not because of spoilers, but because I enjoyed the TV version a great deal more. Fortunately, I had the opposite experience with Tinker Tailor Soldier Spy: having already watched the 2011 film version enhanced my enjoyment of the book.