For mental health and spirituality the Bhagvad Gita. Definitely not what I expected originally and had put it off for years/decades. After reading it, I felt a bit blissed out, what actually led me to read it was another book on taming our mind called The Yogasutras of Patanjali. I feel like it hasn’t been ‘discovered’ yet by the west but it had life changing consequences for me (for the better).
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