A recent thread made me reflect on this. I’ve read all of these, and for the benefit of anyone who would like a reading list of the most foundational spiritual texts.... here you go. I formulated this list according to how universal the texts would be for each spiritual tradition. I chose the traditions based on how widespread the traditions were. There are of course many other important spiritual texts in history both within and beyond these traditions - I am only listing the most universally authoritative.

If you have an ereader, you can download the epub for every one of these texts on z-library for free.

Western texts:
Complete works of Plato
Complete works of Aristotle

(All three of the Abrahamic religions, and several pagan ones, have explicated their teachings through the framework inaugurated by Plato and Aristotle. They are the most universal Western teachers - even more universal than the Torah, because they were authoritative for the Pagans as well).

Hindu texts:
The Principal Upanishads
The Bhagavad Gita

Buddhist texts:
The Pali Suttas
The Pali Vinaya

(After these two collections, Buddhism went off in a thousand different directions. Most of Chinese and Tibetan tradition doesn’t hold these texts in the highest regard, but they are nonetheless still part of their canon, leaving them the most universally canonized texts).

Taoist texts:
The Tao Te Ching
The Zhuangzi