The Bhagavad-Gita is the main source-book on yoga and a concise summary of India’s Vedic wisdom. This is the complete and unabridged introduction as it appears in Bhagavad-Gita As It Is, which is a very good introduction to the philosophy of God, Krishna Consciousness.
Bhaktivedanta Book Trust
The Bhaktivedanta Book Trust (BBT) is the world’s largest publisher of classic Vaishnava texts and contemporary works on the philosophy, theology, and culture of bhakti-yoga. Its publications include traditional scriptures translated into 87 languages and books that explain these texts. The BBT also publishes audiobooks and eBooks. BBT titles range in complexity from brief, introductory volumes and summary studies to multivolume translations with commentary.