Quite a few le Carre novels fall into that category of books to be reread every couple of years. For me it’s "The Spy Who Came in from The Cold", "Tinker, Tailor, Soldier, Spy" and "The Perfect Spy" (perhaps his greatest book) I never get tired of them. This one doesn't fall in that category, but it comes very close.
In German one would say that le Carré is ‘wach’, ie, he really perceives the world around him and has a deep understanding of how its political and power structures work, and how individuals get mixed up in them.
What surprised me the most (or maybe not), was his ability to keep potential action sequences just offstage. Instead he focused on the moral rot and creeping terror. That in itself is quite a feat in my mind.
Last but not least, his use of language, and what use it is. In the dialogue, in the narrative, his willingness to explore moral questions and his evocation of time and place are quite marvelous. Le Carré takes the time to create complex, interesting characters and has the knack to building suspense throughout his stories without relying upon all the usual thriller devices that too many crime authors use. Just for that he always rates high in my "book of wonders".