I would like to nominate The Spy Who Came in from the Cold by John le Carré. I have read Tinker, Tailor, Soldier, Spy and subsequent "Smiley" novels and have always meant to read The Spy Who Came in from the Cold.

From Wikipedia:

The Spy Who Came in from the Cold (1963), by John le Carré, is a British Cold War spy novel that became famous for its portrayal of Western espionage methods as being morally inconsistent with Western democracy and values. The novel received critical acclaim at the time of its publication and became an international best-seller. The novel was selected as one of the All-Time 100 Novels by TIME Magazine.[1] In 2006, Publishers Weekly named it the “best spy novel of all-time”.


I will second The Spies of Warsaw by Alan Furst just because I've grown confident in Issybird's selections. I will also second The Day Of The Jackal by Frederick Forsyth because I read it long ago and recall being spell bound.