Watcher in the Shadows — Geoffrey Household

After an assassination attempt, an ex-spy must run for his life

As far as the police are concerned, Charles Dennim is a zoologist, and there is no reason anyone would want to kill him. And yet, one afternoon when Dennim is working at his desk, death knocks at his door. It is the postman, and he has a package slightly too large to fit through the mail slot. He tries to force it -- and triggers the bomb that lies within. When Dennim emerges from the smoking ruin of his doorway, he sees the innocent postman, ripped in half on his front stoop. The police are baffled, but Dennim is not -- for he was once a spy.
 
This mild-mannered scientist spent World War II embedded deep in Nazi Germany, feeding secrets back to Great Britain. He buried that side of himself long ago, but a nameless killer has decided to dig it back up. To survive, Dennim must remember what it means to be a spy.