I am aware that there are many excellent spy books, but in my modest opinion this is the one that describes the best this strange and dangerous universe that is secret war.
Frederick Forsyth wrote a couple of even better books ("The day of the Jackal", "The dogs of war") but they were not exactly about spying. In this book, written in the 80s, he describes a very clever and dangerous plot of KGB, which, if successful, could really break NATO and leave Western Europe vulnerable to a possible Soviet takeover. The description of Soviet inner circles of power are very good, much better that in the usual spy stuff - Frederick Forsyth was one of the few Cold War spy writers who had a really good understanding of USSR and it shows here. The story is very coherent, the plot is plausible and technically possible and its execution is described in incredibly professional way.
There are however other stories circling around the main plot which describe some of the horribly dirty tricks used by the intelligence communities all around the world (the story takes us from United Kingdom to Soviet Union and then to Africa, before coming back to UK). There are no superheroes in this book (although there are superlosers) and this is definitely not a James Bond movie material - there is however a masterly executed description of the gray, shadowy, dirty and smelly world, where virtually nothing is what it appears and every double bottom has a third bottom... I will absolutely not reveal anything about the plot, but prepare to be surprised. Many times. There are humouristic moments in this book, although this is a dark humour - there is much more tragic fragments, some of which can break the heart.
This is a dark, somber, rare and precious jewel, which didn't age at all since 80s. Excellent to read on holidays or on a rainy weekend. Once you finished it, a good idea could be to read some other early Forsyth books, like "The Day of the Jackal". If by any chance you saw already the movie, with Pierce Brosnan and Michael Caine, please be aware, that although a honest effort this film was only a shadow of this great book.