The Prisoner in the Castle by Susan Elia MacNeal (Maggie Hope series Book 8)

This is the eighth and final book in the Maggie Hope series

Summary
-World War II is heating up. Maggie Hope is a spy with too much knowledge. She knows too much for the British government to sit by idly. She knows where in France the Nazis are going to invade. Maggie is on a remote Scottish isle called Killoch Castle. Her fellow agents are dropping like flies. One even drops in the middle of his after-dinner drink, making his death just the first of many. Maggie must investigate before she herself becomes the next victim.