The Heat of the Day by Elizabeth Bowen

It is wartime London, and the carelessness of people with no future flows through the evening air. Stella discovers that her lover Robert is suspected of selling information to the enemy. Harrison, the British intelligence agent on his trail, wants to bargain, the price for his silence being Stella herself. Caught between two men and unsure who she can trust, the flimsy structures of Stella's life begin to crumble.