This is a period in the history of india dating to as far back as 1200BC. Four vedas were written (Rig, Yajur, Sama, Atharva). The earliest and most important is the Rig Veda. They were originally a collection of hymns, but it is believed at a later period the religious precepts of the brahmanas caste were then appended.

The mantra or Samhita part of the Vedas is thought to be a product of the Aryans, who were a tribe from Central Asia that moved both to Persia and India. They may have been collected when the Aryans met resistance from indigenous indian who wanted to define their culture more strongly during a period of conflict. There is also the Kurukshetra War which arose as a dynastic struggle between two groups for control of a kingdom known as Kuru, although precise dates more this conflict are difficult to pin down.

The main issues in early Indian philosophy however does not appear to be the the acceptance of the Vedas, but around the cultural acceptance of the authority of the Brahman caste. The groups that rejected the Brahmins became regarded as unorthodox. These were groups such as Charvakas, Jains, Ajivikas and later Buddhists, all of which achieved large measures of success as faiths at different times. The growth of these other groups appears to have also caused some reform in the orthodox hindu culture allowing some low caste hindus to later be allowed greater study and participation rights in society. The later Upanishads with sages such as Shankara and Ramanuja were commentators on the earlier Vedas and developed key orthodox ideas further.

A good book is "Eastern Philosophy" by Kevin Burns 2004