Showing The Key by Patricia Wentworth

The murder of a government scientist demands the attention of Miss Silver.

Michael Harsch's life has never been easy. A German Jew, he fled his country when Hitler came to power, escaping the concentration camps by the skin of his teeth. His wife and daughter were not so lucky, and he vowed revenge on the Fuhrer through science. He set to work on a marvelous new explosive that, in the hands of the British army, could silence the German guns forever. But on the eve of his great triumph, the scientist is struck down.

The government asks Miss Silver, the dowdy detective, to help solve the murder and recover the valuable explosive. Was Harsch killed by a half-mad opponent to the war effort, or was it one of Hitler's undercover agents who pulled the trigger?