Religion? That's right! You gotta study religions (or philosophy)!
Recommended reading:
Bhagavad-Gita as it is: https://vedabase.io/en/library/bg/
Zoroastrianism is also interesting: http://www.avesta.org/
The Bible (at least read the Gospels).
Early Christian writings are also interesting: https://www.earlychurch.org.uk/
So are the writings of Church Fathers: https://www.newadvent.org/fathers/
How about some heresy? Read about the various Gnostic groups: http://www.gnosis.org/naghamm/nhl.html
Here is a link to a site that contains a lot of classics (like Plato): http://classics.mit.edu/index.html
The Sacred Texts website is also full of various texts: https://www.sacred-texts.com/
There is also the Esoteric Archives website: http://www.esotericarchives.com/
Bonus site: http://english.grimoar.cz
Bonus site2: https://www.grymoire.com/ (this site contains content for both UNIX wizards and other kinds of wizards)
Also check out the "Reading Recommendations" thread and the "Books" thread.