Ever since reading "The Secret Adversary" I've been a fan of Tommy and Tuppence. They bring humor and sillyness into the world of mystery and suspense.
In N or M we see this again. Tuppence refuses to be placed into any sort of stereotypical feminine role, but does so in a manner that that delights me as a reader. Tommy is with her all the way. Like he says: "from the first we were a joint venture". When he encounters her at Sans Souci (the suspected headquarters of the fifth column), he is delighted and awed. When his boss discovers that they were both tricked he laughed.
It turns out (of course) that it is a good thing that Tuppence is there. Without each other they would not have been able to solve the mystery. There are no clear suspects. Like all Agatha Christie books (and perhaps all others of her genre) first one then another person is suspected of being the culprit. The ending is not surprising but unexpected nevertheless.
This was a terrible time in Europe. Things were black and white and the propaganda at that time in GB was very anti-German, understandably so. In many ways this book shows how group mentality can work and how our attitudes are affected by what is shoved in our faces every day - elementary psychology.
I guess this one thing I really like about Christie's books. She shows how people are and how they think. The culprit is generally just any old person living a normal life, but who then manages to encounter extraordinary circumstances. Sure, there are mass murderers and crazy people in her books as well, but in general the culprit might just as well have been my neighbor.
Enjoy! I certainly did.