Bhagvad Gita - "Song of God" . It is a part of epic Mahabharat. In which Krishna who is incarnation of God himself is explaining about secrets and purpose of life to his friend and devotee Arjuna.
"If truth is what works, as Pierce and pragmatist insist there must be a kind of truth in Bhagvd-Gita". It is deeply felt, powerfully adapted and more quoted jwell for mankind.