The Polish Officer by Alan Furst is an excellent spy novel set in WWII.
September, 1939. The invading Germans blaze a trail of destruction across Poland. France and Britain declare war, but do nothing to help. And a Polish resistance movement takes shape under the shadow of occupation, enlisting those willing to risk death in the struggle for their nation's survival. Among them is Captain Alexander de Milja, an officer in the Polish military intelligence service, a cartographer who now must learn a dangerous new role: spymaster in the anti-Nazi underground.
Beginning with a daring operation to smuggle the Polish National Gold Reserve to the government in exile, he slips into the shadowy and treacherous front lines of espionage; he moves through Europe, changing identities and staying one step ahead of capture. In Warsaw, he engineers a subversive campaign to strengthen the people's will to resist. In Paris, he poses as a Russian poet, then as a Slovakian coal merchant, drinking champagne in black-market bistros with Nazis while uncovering information about German battle plans. And a love affair with a woman of the French Resistance leads him to make the greatest decision of his life.....