Although this is a children's book, it has a very interesting concept.
The book is about a young American boy, who travels to France to stay with his uncle in a small town. It is post-world war II. As he explores the town and make new friends, he comes to realize that there is a Nazi spy hiding in the town, who also seems to hold a personal vendetta against him.
It depicts the thought process of a stubborn young boy who transforms in to a completely different personality as the town challenges him with unusual life experiences.

THE BEST PART - When the boy mingles with the French people, he starts learning the French language. As the story progresses, we are introduced to a lot of French words and sentences. Before you realize, you are already reading a French book. The last three pages are in complete French. Believe it or not, you WILL understand every single word, even if you have never learnt French before.

This concept is so interesting. I wish to read such books for a lot of other languages or dream to write one, for Hindi or Gujarati.
It is a very distinctive way to learn a new language.