The Man Next Door — Mignon G. Eberhart

Set during WWII and featuring a female sleuth, “a good spy story with overtones of romance; tense situations and excellent characterization” (The New York Times).

Maida Lovell takes her job as secretary to the US War Department head, Steve Blake, seriously. So when a routine assignment to pick up some notes at his family's home has her stumbling upon a body, and Steve sure to be the chief suspect, Maida agrees to a coverup to protect her boss.

But the tables quickly turn and suddenly Maida is at the mercy of an enemy spy intent on blackmailing her into uncovering national secrets. Now, Maida must do all she can to save herself -- and her country -- from disaster.