The oldest scriptures of India, the Vedas (literally "knowledge," or "sacred teachings"), are a vast collection of Sanskrit hymns, prayers, and rituals, composed between the fifteenth and tenth centuries B.C. These works are divided into four books: the Rig Veda, the Veda of hymns, which praises the gods, the elements, and the bounty of earth; the Yajur Veda, the Veda of sacrificial texts; the Sama Veda, the Veda of songs; and the Atharva Veda, the Veda of Atharva, overseer of the sacred fire ceremony. The Rig Veda is the first Veda-and the oldest existing scripture of India.

At the end of each Veda is a section called the Upanishad which literally means, "sitting close, with devotion." They were composed between 1300 and 800 BCE. Etymologically, the word suggests sitting at the feet of an illumined adept in an intimate session or spiritual instruction. This final section of each Veda contains the "secret teachings" of the ancients, which, over the centuries, have been well-guarded and meant for the most worthy disciples. The Upanishads inquire into the nature of reality, God, and the universe. They are more probing than the rest of the Vedas, and they address the essential issue of how a seeker can attain God-Realization. The Upanishads teach that the Absolute Reality, Brahman, dwells fully in the human soul; and that this Reality is meant to be known. The teachings of the Upanishads are called Vedanta, which means "at the end of the Vedas."

Source : Jonathan Star, The Inner Treasure

Rig Veda 10.32.7 - Translation Ralph T.H. Griffith
Mundaka Upanishad - Translation by Eknath Easwaran
Katha Upanishads - Verses 2-1 & 2-2
Prashna Upanishad - Translation Eknath Easwaran
Isha Upanishad Verses 4-7 - Translated by Eknath Easwaran
The Shvetashvatara Upanishad - Translation Eknath Easwaran
Hidden In All Things - Upanishads
Nothing But The One - Chandogya Upanishad 7.23, 27
Brihadaranyaka Upanishad IV.4.5 - Translation Eknath Easwaran