N or M? is the third of the Tommy and Tuppence stories, set in 1940 and first published in 1941.  Agatha Christie wrote this at the same time as writing The Body in the Library. She explained the reason in her Autobiography:

I had decided to write two books at once, since one of the difficulties of writing a book is that it suddenly goes stale on you. Then you have to put it by, and do other things – but I believed that if  I wrote two books, and alternated the writing of them, it would keep me fresh at the task. One was The Body in the Library, which I had been thinking of writing for some time, and the other one was N or M?, a spy story, which was in a way a continuation of the second book of mine, The Secret Adversary, featuring Tommy and Tuppence. Now with a grown-up son and daughter, Tommy and Tuppence were bored by finding that nobody wanted them in wartime. However, they made a splendid come-back as a middle-aged pair, and tracked down spies with all their old enthusiasm. (An Autobiography by Agatha Christie page 506)

Tommy is asked to go under cover to track down members of the Fifth Column, two of the most important and trusted German agents, whose mission is to infiltrate British society, like the Trojan wooden horse. All that is known is that N is a man and M a woman and they are thought to be at Leahampton on the south coast. He tells Tuppence that he is being sent to Scotland and that she can’t go with him, but she surprises him by being at Sans Souci, a seaside guesthouse in Leahampton, when he arrives. So there they are, both under cover, Tommy as Mr Meadowes and Tuppence as Mrs Blenkensop.

There is definitely something not right about the guesthouse, it has the feel of something sinister, something evil. And it’s not long before Tommy and Tuppence are embroiled in a series of dangerous near-disasters, involving German spies, and Smuggler’s Rest, a cottage with a secret room, set on a cliff overlooking a little cove and ideal for enemy action.

N or M? is an easy book to read and not too demanding. Agatha Christie makes use, as in some of her other books, of nursery rhymes, in this one it’s ‘Goosey goosey gander’, which comes from a Mother Goose picture book Tuppence reads to little Betty Sprot. Of all the characters in the book (apart from Tommy and Tuppence) Betty, a toddler, who speaks her own baby language, is the most well-drawn, so much so that at one point I even found myself wondering if she could be M!!!

One of its attractions for me is its historical setting, although when Agatha Christie wrote this book it was very current, she did not know how the war would end. It is interesting to see how she portrays the general public’s attitude towards the war, about patriotism, and the fear of Fifth Columnists, of spies, and Fascists and Communists. Also of note is that whilst most of the characters thought the war would be over very quickly, which is what I thought was the general consensus at the time, one of them thought it would last at least six years.

Following the publication of N or M? Agatha Christie was investigated by MI5 because she had named one of the characters ‘Major Bletchley’ and MI5 suspected she had a spy in Britain’s undercover code breaking centre, Bletchley Park.