This is the quintessential text of the tradition of Hinduism. It is undoubtedly one of the more fun religious books to read.
Arjuna is a price from the Kshatriya (warrior) caste, faced with a dilemma. He is tasked with fighting in a way he doesn't want to fight. A rift in his clan has left him in the position of having to engage in battle with his own flesh and blood.
Distraught by the war at hand, he consults with his charioteer Krishna for advice and emotional support. Krishna, who is actually god in human form, addresses these concerns while secretly trying to lead the warrior to complete Enlightenment.
The Bhagavad Gita is a series of dialogues between Arjuna and Krishna distilling ancient wisdom on how to conduct oneself in life.