1) Classic Books About the Spiritual Path
The Bhagavad Gita.
There are many versions but the versions by Swami Prabhupada and Stephen Mitchell are excellent.
The Tao te Ching
I find the version by Stephen Michell to be poetic and clear.
The Ashtavakra Gita.
An ancient spiritual text aimed at triggering direct realization of the true Self. Here is a link to the PDF version.
The Kena, Maitri, Mandukya, and Taittiriya Upanishads are a good place to start.
The Secret of the Golden Flower
There is a version by Richard Wilhelm and Carl Jung, and another version by Thomas Cleary in which he corrects mistakes in the previous version. Both versions are valuable.
2) Books About Non-duality
“Be as You ARE”
“The First and Last Freedom”
Sri Nisargadatta Maharaj
“I Am That”
3) Books related to Soul Exploration/ Developmental Aspects
“The Red Book”
“The Way of Initiation”
“Spiritual Body and Celestial Earth”
4) Integrating the Awakening Experience into Life
“The End of Your World”
“A Mind at Home with Itself”
“A thousand Names for Joy”
“After the Ecstasy, the Laundry”
5) Big Picture/ World Evolution
“The religion of tomorrow”
“A New Earth”