My all-time favorite "spy" book is Eye of the Needle by Ken Follett. It is legitimately great writing and a debut novel to boot. Follett later wrote the Century Trilogy which I also found deeply engaging and enjoyable- it's a "gone with the wind" treatment of the 20th century through the perspectives of a British, a Russian and an American family. I suspect that everyone who loves Herman Wouk loves him because Follett had yet to write the Century Trilogy. Second and third place would be Day of the Jackal and Dogs of War by Frederick Forsythe. There was a time when friends of mine decided we were going to set ourselves up as despots in a small African nation. Chinese guns and ammo were cheap. We settled on a force of 1500 and overthrowing Togo. Six months later I discovered that Frédérick Forsythe had written a book about mercenaries overthrowing a small African nation for profit... And for research, set about with his buddies to overthrow the government of Togo.