Agatha Christie - N or M? (Complete and unabridged)

N or M? is almost a direct link on from 'The Secret Adversary', except in the Second, rather than the First World War. No matter - the setting is perfect and the characters drawn in wonderful Christie style. Also, unlike the later two novels (By the Pricking of my Thumbs and Postern of Fate), there is actually a mystery to solve and enough clues to get it.