Regarding the level of language, it seems that this is a book written for children. But when you focus on the content and the message of the book it is clear that it is rather a book for adults. There must be a certain age to see how the author plays with prejudices towards happiness and disenchantment with Hector when he has to realize that the happiness he believed, does not really exist.
In the end, I was fascinated by this book, especially a few lessons of happiness (p.ex.Le sun and the sea, is happiness for everyone.) And I'm glad I read " The trip Hector 'myself instead of just read other reviews online.
