This book deserves to be better known among those who enjoy espionage novels, especially those set in the Cold War. It is a masterfully written, beautifully experienced narrative. In the end, it is much more than just another spy story. Absolutely at the top of any list.
The author's later books, while quite good, never manage to repeat what he pulled off in this one. That's no criticism of him or of them, but rather an appraisal of what his first book accomplished.