What is a book/essay/article that you keep going back to for learning about a fundamental thing?

Fundamental thing implies self exploration, something timeless about markets, evergreen content.

I am collecting some of these articles for myself and if I have to select only one, that would be this

P.S. I would prefer that everyone who answers this chooses one piece of content that they think has/had the most impact.

The book Antifragile by Nassim Taleb, for learning about life. Hits me differently every time I read it. Because I'm in different life phases or have more / other knowledge.

I've never read a book twice yet, but I really liked this very short (~1-hour) book:

Steal Like an Artist, by Austin Kleon

Beelzebub's Tales to His Grandson by George Gurdjieff. I find it perfect to learn about hermeneutics by practice.

You don't need to become super adept on reading deep theological / academic stuff after it, but it's useful to learn to interpret everything as units with many layers of meaning (ex. the news, history articles in certain language by a certain country...).

It's written obfuscated on purpose: it appears to be a sci-fi book with "random" "invented" words to describe Humanity from the perspective of an alien, but it's a philosophy book about Human psychology with heavy references to different world religions and cultures.…

btw thank you for sharing the piece about ego!