Most Secret by Nevil Shute

Most Secret is set during World War II and follows the story of a British scientist named Dr. Philip Raven, who is working on a top-secret project to develop a new type of explosive. Raven is also a double agent, working for the British intelligence agency while also providing information to the Germans. As he carries out his mission, Raven must navigate the dangerous waters of espionage and counter-espionage, while also dealing with the moral complexities of his actions. Along the way, Raven falls in love with a woman named Helen, who becomes embroiled in his mission and must also make difficult choices about her own loyalties. "Most Secret" is a thrilling tale of espionage and wartime intrigue, and a thought-provoking exploration of the costs and consequences of individual actions during times of war.