Pursuit of a Parcel (The Ernest Lamb Mysteries)

Enemy forces will stop at nothing to retrieve an incriminating package in this gripping international thriller from the author of the Miss Silver Mysteries
 
Double agent Cornelius Roos is about to become dispensable to the Germans—until he reveals the existence of a recording that will guarantee the death of a high-ranking Nazi official if it finds its way into the hands of Hermann Goering. So Roos strikes a bargain: If he walks out of Gestapo headquarters alive, he will ensure that the compromising recording never reaches Goering.
 
Meanwhile, in the Foreign Intelligence office in England, agent Antony Rossiter is interrogated about Roos, his older, adopted brother. A few days later, Rossiter parachutes into Holland to make contact with Roos. But when a brown paper parcel with Rossiter’s name on it is delivered to a British law firm, Rossiter’s fiancée, Delia Merridew, becomes an innocent pawn in a deadly game of international espionage and cold-blooded murder. Now it’s up to Scotland Yard Inspector Ernest Lamb and Detective Frank Abbott to ferret out the truth before a desperate enemy claims another victim.
 
Pursuit of a Parcel is the 3rd book in the Ernest Lamb Mysteries, but you may enjoy reading the series in any order.