The Key To Rebecca by Ken Follett

The novel is a gripping World War II espionage thriller set in North Africa, where an elusive German spy, armed with a powerful secret code named after a classic novel, is transmitting critical information to Rommel's Afrika Korps in their campaign against the British. The British intelligence is in a desperate race against time to intercept and decode the messages, and they deploy a resourceful and determined young officer to track down the spy. Amidst the backdrop of war-torn cities and treacherous desert landscapes, a deadly game of cat and mouse ensues, with the fate of the North African campaign hanging in the balance. The 6320th greatest book of all time