Have you seen the movie? I did, and before I read the book. Memories of the film flooded my reading experience. The novel includes lots more detail and expands the cast. I enjoyed the film more because the reveal--who is the spy?--is done with greater effect. And of course, the Julio Iglesias overdub at the end is magnificent.
Tinker, Tailor, Soldier, Spy is the story of a forcibly retired senior officer of British intelligence, George Smiley, getting informally recruited back into service. His mission is to identify a Soviet mole in the head office. The story is a study in the play between loyalty and identity. I found the narrative thread difficult to follow in both film and print. Fans of the film who have never read the book can do without the read. But my opinion is that the reverse is not true.
Note:
The British spy jargon created problems for me. A lexicon appendix would have helped.