It is, of course, a trilogy, but I wouldn't be inclined to read 'The Honourable Schoolboy', just jump from 'Tinker, tailor...' to 'Smiley's People'. The Honourable Schoolboy is a decent read, but is a little too long and convoluted for my tastes in spy-fiction. On saying that, referring to these books as 'spy fiction' is a bit like saying Graham Greene writes decent thrillers. These books of Le Carre's are elegant, often poetic, as much about the human condition as about the Cold War. Smiley's People is a most satisfying conclusion to the story - the books starts from two seemingly unconnected events and the narrative wonderfully brings things to a conclusion in relative brevity.
I do not actually like George Smiley, retired spy-master. Not the character as written, but the man himself. And being able to make the distinction between the two, I think, is a mark of great writing.