This invaluable study of the history of the text of the Gita is unfortunately out of print, but a must read for anyone interested in unraveling the enigmas of Indian spiritual history and wresting the gold from the illusions here. Attempting to show that the book we now have is a very late version from the period ca. 800 A.D. with a theistic cast not present in the original, the book also uncovers the connection with the Samkhya tradition, a very early form of which was probably part of Gautama's path. As if the text of the Iliad had fallen into Christian revisionism and made a document of spiritual monotheism, the Gita that we have is an eloquent but misleading series of confusing layers. The author also suggests certain interpolations in the text of the Yoga Sutras designed to make it seem compatible with the tide of monotheism sweeping India in the later medieval period.
Cf. also a work such as Gerald Larsen's Classical Samkhya, for the difficult history of this uniquely interesting yoga, and its tortuous history.

Sinha argues that the original Gita, which was based on the Samkhya and Yoga philosophies was much shorter (86 verses). The rest of the Gita are later interpolations done by a monists who wanted to further the interests of the priestly class (brahmins). The work is well researched and has many references. I am sure that the orthodox folks with a closed mind aren't going to like this book.

The title says theme of the book: The GITA as it was : Rediscovering the original Bhagvadgita. His basic motivation comes his analysis of indian social condition as he has pointed in his preface. The key argument is that there was an original gita of 84 verses written by Vyasa, which is consistent in basic theme with Kapila's Samkhya Darshan (700) and Patanjali's Yoga sutra (400 BC). The Bhagavagita of the modern form is , in his angle, intentionallys construted by Brahmins in the period of 800 AD. It borrowed themes of monothesim, hell, heaven, sin and salvation themes from Christanity and Islam. He projects that Shankaracharya is the key person in this reformulation. Given the fact many scholar both Indian and Western agree that mahabharata has through three evolution jaya, vijaya and mahabharata, the basis to look to look for similar evolution is reasonable. Earlier Khair, G.S. has identified three kala geeta based on the motivation and conceptual analysis of the content of Bhagadgita. Sinha's better approach is better since he looks for the original Samkhya Karika and yoga sutra, in which he has success. He draws evidence for his stand from Alberuni's discussion about Gita and a text of Gita of 80 verses found in Indonesia, written in Palm leaves in the Kavi (Balinese) language.