Although Agatha Christie is mostly known for her murder mysteries, mostly starring Hercule Poirot and Miss Marple, she was also a very good espionage writer. Those books are few in number, but one of my favorites is N or M?, which features Tommy and Tuppence Beresford. Tommy and Tuppence were only in a handful of Christie books: The Secret Adversary, Partners in Crime, N or M?, By the Pricking of My Thumbs, and Postern of Fate, which was the last book that Christie wrote. Tommy and Tuppence start out as young people fresh from World War I with no prospects. They get involved in a spy case, looking for the mysterious Jane Finn. They open a detective agency in Partners in Crime, which is a collection of short stories with an overall unifying theme of working for Mr. Carter and looking for a man known as No. 16. For each case in the book, Tommy and Tuppence took on the personality of a well-known (in 1929) fictional detectives. With N or M?, Tommy and Tuppence are now middle aged. World War II is in full force, and the Berefords want to get in on the act of helping their country, but they are dismissed as being "past it." War is for the young, like Tommy and Tuppence's twins, Derek and Deborah.
N or M? starts with both Tommy and Tuppence down in the dumps because they don't have a role in the war. When someone comes knocking at their door, we all know that it is adventure. This time, it's in the shape of Mr. Grant, who has been sent to solicit the services of Tommy for a boring desk job in Scotland. When Tuppence gets called away to tend to a sick friend, Mr. Grant lets Tommy know that Scotland is a rouse. He's really going to a boarding house in Leahampton called Sans Souci to look for a Nazi spy. The spy can be one of two main people known as N and M. N is a male, while M is a female. Tommy is not to tell Tuppence a thing, and a few days later, he goes off to "Scotland." Imagine Tommy's surprise when he arrives at Sans Souci, as Mr. Meadowes, to find Tuppence already in residence as Mrs. Blenkensop, Tommy is elated. Tuppence figured out that Mr. Grant didn't want to talk in front of her, so she plotted the phone call with a next door neighbor. As Tommy and Tuppence investigate the inhabitants of Sans Souci, they realize that people are not as they appear. Just as Tommy and Tuppence are playing a role, any of the other house guests could be someone else. The only one who seems above suspicion is Mrs. Sprot with her two-year-old daughter, Betty. Christie cunningly hides the identities of N and M, and I was greatly surprised by both reveals. There are no murders in the book, but there is lots of adventure.
I loved N or M?, and it makes me want to re-read the first two Tommy and Tuppence books. The latter two are not very good, so I'll just avoid them for now. As Christie would say, you are not past it until you are dead, and you should never believe everything people tell you.
Keep your eyes open for the part where Christie has a character talk about amnesia that is brought on by the stress of life. This is obviously an reference to the amnesia adventure that Christie had. She rather famously went missing in 1926 when her husband Archie Christie was divorcing her. The claim was that Christie suffered amnesia from the stress of the divorce. You can read more about that here.