'Eye of the Needle' is an early novel from Ken Follett, now best known for his doorstep historical sagas. The book was first published in 1978, by which point Follett had already published a bunch of crime novels and trashy tie ins. It was his first big hit and was filmed 3 years later with Donald Sutherland in the lead role. 
It's not hard to see why it was so successful. It's a gripping, convincing thriller set in WW2 Britain, and follows a German spy who has discovered the truth of the D-Day preparations. Pitched against him are the authorities (both MI5 and the police) and some civilians who get caught up in events. 
What impresses most about it is the character work and the sense of place. It really does feel like an accurate portrayal of life in wartime Britain, and the cast (including trhe spy himself) are convincingly and sympathetically drawn. You know he has to be stopped, you want him to be stopped, and yet you also feel the tension of his double life. 
It's all great, page-turning stuff, and very enjoyable.