The Little Prince [suggested by Phil Howarth]

The Little Prince is a classic tale of equal appeal to children and adults. On one level it is the story of an airman’s discovery, in the desert, of a small boy from another planet – the Little Prince of the title – and his stories of intergalactic travel, while on the other hand it is a thought-provoking allegory of the human condition.

First published in 1943, the year before the author’s death in action, it is apparently the most translated book in the French language. Both moral fable and spiritual autobiography, the little boy lives alone on a planet not much bigger than himself, and leaves it to travel round the universe. [product description adapted from Amazon]

The book has a Wikipedia entry

Author’s Wikipedia page
Author’s entry on biography.com