Crime Fiction Alphabet: N is for N or M? by Agatha Christie

I read N or M? long time ago. All my Agatha Christie novels are pretty old. My copy has the cover shown here, published by Fontana in 1976. I am thinking of going through my collection again. Currently reading The Moving Finger. Thought I would showcase N or M? for Crime Fiction Alphabet. I don't recall much of it other than it being a Tommy and Tuppence novel!

Here is the synopsis from http://www.biblio.com/agatha-christie/n-or-m~36412941~title

This atmospheric story, set during World War Two, finds Tommy and Tuppence doing what they do best – having an adventure. These unlikely Intelligence Service spies decide to help Queen and country by tracking down two ruthless traitors. The only clue to the traitors' identities is a government agent's dying words that lead them to the Sans Souci boarding house, where it'll take some extremely subtle detection work to establish which of the guests are the treacherous N and M.Published in 1941, the novel is one of two which Christie worked on simultaneously during the Second World War whilst living alone in London. She alternated her writing of The Body in the Library and N or M? to keep herself "fresh at task."