The Secret Adversary
Tuppence appears as a charismatic, impulsive, and intuitive person while Tommy is less imaginative and less likely to be diverted from the truth (as their first adversary sums him up: "he is not clever, but it is hard to blind his eyes to the facts") which is why they are shown to make a good team. It is in the first book "The Secret Adversary" that they meet up after the war, and come to realise that, although they have been friends for most of their lives, they have now fallen in love with each other.