The Ministry of Unladylike Activity by Robin Stevens TPB

1940. Britain is at war, and a secret arm of the British government called the Ministry of Unladylike Activity is training up spies. Enter May Wong- courageous, stubborn, and desperate to help end the war so that she can go home to Hong Kong (and leave her annoying school, Deepdean, behind forever). May knows that she would make the perfect spy. After all, grown-ups always underestimate children like her. When May and her friend Eric are turned away by the Ministry, they take matters into their own hands. Masquerading as evacuees, they travel to Elysium Hall, home to the wealthy Verey family - including snobby, dramatic Nuala. They suspect that one of the Vereys is passing information to Germany. If they can prove it, the Ministry will have to take them on. But there are more secrets at Elysium Hall than May or Eric could ever have imagined. And then someone is murdered...