Rigveda is the oldest book of the world.With the time of its construction, historiests are not agree totally.But generally it is considered between 3000 to 2500 bc.
It is said that there were 21 branches of Rigveda,but nowadays there are only 5 branches available.There are- Shâkal, Åshwalâyan, Shânkhåyan, Vâkal, Mândukâyan.