I had tried watching the recent movie adaptation of Tinker Tailor Soldier Spy and was so put off that I had really low expectations when I happened into a copy of A Perfect Spy.

I was blown away by reading APS, I think it's really a masterpiece and would recommend it to anyone who has ever had a family, a job, or a secret.

I've since read most of the rest of his work and while APS is the best, the Smiley series was also very good and I felt genuinely moved by the conclusion of Smiley's People. You don't really have to read the rest in order but read Smiley's People last.