I just finished Smiley's People, the third in the Karla trilogy by John Le Carre (or David Cornwell). It is an amazing series of books. I tried thinking of which book is my favorite.
Tinker Tailor is a great story, and appealed to my desire for suspense in a spy novel, but also showed the extreme psychological side of that sort of thing.
The Honorable Schoolboy was a great story. It took my a while to get into it, but once I knew the plot it fully engrossed me. There was quite a bit more action in that one, with most of the emphasis being on the field man Westerby and not the case man Smiley. Still more emphasis on the moral ambiguity of the situation instead of blind adherence.
Smiley's People returns to George and his ways of thinking. I also enjoyed how most of the characters in the past two came back for at least an appearance. The end is also brilliant. I probably went through this one the fastest of all three.
The Spy Who Came In From the Cold is up next. Then I might go back and read the early Smiley novels. I highly recommend Le Carre's novels to anyone who enjoys that sort of thing. Finally a spy novelist who is a good writer and not just a good storyteller.