The Bhagavad Gita is technically part of Book 6 of the Mahabharata, although it is known to be a later accretion to the saga, which stands on its own merits. It is a dialog between the God Krishna and the hero Arjuna, taking place in a timeless moment on the battlefield before the climactic struggle between good and evil. The Gita (which can be found in hotel bedstands throughout India) is a classic summary of the core beliefs of Hinduism. It has had a significant influence far beyond Hinduism. Robert Oppenheimer apocryphically recited the verse (from Chapter 11) 'I [have become] Death, Destroyer of Worlds", just before the first test of the atom bomb, which ironically has a much different meaning in context.
This has now been cross-referenced with the Sanskrit text.