A Presumption of Death — Jill Paton Walsh

While Lord Peter is abroad on a secret mission, Harriet Vane, now Lady Peter Wimsey, takes their children to safety in the country. But there's no escape from war: rumors of spies abound, glamorous RAF pilots and flirtatious land-girls scandalize the villagers, and the blackout makes rural lanes as sinister as London's alleys. And when a practice air-raid ends with a young woman's death, it's almost a shock to hear that the cause is not enemy action, but murder. Or is it? With Peter away, Harriet sets out to find out whodunit...and the chilling reason why.