The Little Prince
The Little Prince is an all-time classic. Antoine de Saint-Exupery said it was a book for children written for adults - and it can definitely be enjoyed by both. The book has been translated into over 200 languages so if you are learning a foreign language, it is a useful idea to buy a copy in that language - good for your vocabulary and grammar, easy to understand (since you have already read it in your own language) and provides a great sense of achievement once finished.
Just in case you haven't already read the book, it is a charming story of a little boy who leaves his own tiny planet to travel the universe. He has a series of encounters which help him to learn about the weirdness of adult behaviour. His odyssey culminates in a voyage to Earth and further adventures ensue.
No more detail or the pleasure of meeting The Little Prince for the first time will be ruined and so will the fun of re-reading after an absence of many many decades.