Srimad Bhagavad Gita -"as it is" by Gita Press.
Understanding Guru Granth Sahib : The Living Guru by Satjit Wadva.
Introduction To Vedanta By Swami Dayananda.
Ashtavakra Gita by Sri Sri Ravi Shankar - Only for those who are at a very advanced state of consciousness.
Light on Patanjali yoga Sutras.
Yoga Vasistha - Rama Gita.
The Lotus Sutra.
The Holy Bible.
Vigyan Bhairav - The Science of Consciousness.