All of it. You can't really understand anything without the foundations. Truth, justice, morality, human nature. Incentives. Systems. Money, consequences. Men and women. I mean, it's just life. It's everything. It's the Bible.

Maybe start with these

- Matthew

- John

- Luke

- Mark

- Romans

- Proverbs

- Sirach

- Wisdom

- Psalms

- Ecclesiastes

- Acts

I don't think most modern people (including me for decades) have any clue what the bible is or what is in it. There is really nothing else like it. I recommend the Douay Rheims, which is the original translation of the Latin Vulgate into English, which was the translation of the Septuigent into Latin from the Greek (by St. Jerome), so that's the pure original thing.