Kinda cliché but The Little Prince. I read it for the first time when I was about 13 and remember being mocked at because it was a "book for little kids". I could not care less, since I was already pretty sure that kids wouldn't understand the meaning at all, but I never imagined how badly I would want to read it again (which I did multiple times ever since). I really think if we all read it at least once a year we would improve a lot.
