Perhaps you have heard of this, perhaps this is the first time. Bhagavad Gita means, in Sanskrit, "the Song of Our Lord;" the text is a 700 verse song, with 18 chapters.
The Bhagavad Gita was first spoke millions of years ago, Krishna explains, to the sun god, Visisvan. He explains that He descends whenever there is a decline in religious practice, to annihilate the demons and to establish the devotees.
The Gita is the richest source of spiritual wisdom of which I know, equal to all. There are interpreters who say that it is only a book of wisdom, and the statements about Krishna being God are meaningless, but I don't know what their motives are.
His Divine Grace A.C. Bhaktivedanta Swami was a devotee of Krishna who explains that Krishna is God because he fits the description. No one else is God, nor did Krishna become God. Krishna is God because he possesses the six qualities of supreme beauty, wealth, knowledge, renunciation, wisdom, and power. We are similar in that we have these qualities, but not to the same extent Krishna has.
The Bhagavad Gita is holy because it is the instructions of Krishna. Krishna explains the purpose of life is yoga, and the perfection of life is devotional yoga, or bhakti yoga; more specifically, service and devotion to God.
I've only read the Gita a few times, but I've discussed it a lot and chanted a lot; which is the prescription sacrifice for spiritual development in this age. I've attended lots of Bhagavad Gita classes; what I mean is I would love to open discussion of this holy book and I am most happy to give answers about the philosophy.