The Amateur by Robert Littell

A spy thriller classic, now a major motion picture starring Rami Malek, Rachel Brosnahan, Caitriona Balfe, and Laurence Fishburne.

Charlie Heller is an ace cryptographer for the CIA, a quiet man in a quiet back-office job. But when his fiancée, Sarah Diamond, is murdered by terrorists at the American Embassy in West Germany and the Company refuses to pursue her killers, Heller takes matters into his own hands.