From Wikpedia
The Mahabharata (Devanagari: महाभारत, phonetically Mahābhārata - see note), sometimes just called Bharata, is one of the two major ancient Sanskrit epics of India, the other being the Ramayana. Traditionally ascribed to Vyasa, it is the longest literary epic poem in the world. Besides being hailed as one of the greatest literary accomplishments of humanity, it is also of immense religious and philosophical importance in India. In particular, the Bhagavad Gita, which is one of its chapters (Bhishmaparva), is a central sacred text of Hinduism.
My parents immigrated from India in the late 50s. Both of them were (in my mother's case is, my dad died last year) nontheists, however they both wanted my siblings and me to understand and appreciate our heritage.
So we read all the myths and religious texts as well as basic history. The Mahabharata remains my favorite book to this day. I've started reading it to my son (he is young so a lot of it is lost, but it has kings and battles galore to keep his interest)
The story in a nutshell is the struggle for a kingdom between cousins, the Kauravas and the Pandavas. The Bagavad Gita is part of this story, Arjuna (a Pandava) doesn't want to take arms against his cousins (The Kauravas) but is convinced by Krishna it is his duty (Krishna reveals himself as god).
Other myths and stories are interwoven in to the story in the form of character narratives (one character telling another a story to make a point, etc.)
As a child I read the comic book versions, as I got older I read various translations. My favorite translation isn't available here, but you can find many others at Amazon (or elsewhere).
edited to add: This is fiction and I just realized this is the non-fiction area (D'OH!!), so it may not be what you're looking for!