Princess Elizabeth's Spy

Description for PRINCESS ELIZABETH'S SPY by Susan Elia MacNeal:

As World War II sweeps the Continent and England steels itself against German attack, Maggie Hope, former secretary to Prime Minister Winston Churchill, completes her training to become a spy for MI-5. Spirited, strong-willed, and possessing one of the sharpest minds in government for mathematics and code-breaking, she fully expects to be sent abroad to gather intelligence for the British front. Instead, she is dispatched to go undercover at Windsor Castle -- to tutor the young Princess Elizabeth in math. Yet the upstairs-downstairs world at Windsor quickly proves more dangerous -- and deadly -- than Maggie ever expected.

This is #2 in A Maggie Hope Mystery.