Riga Veda
Riga Veda is considered the oldest among the four Vedas. It is subdivided into ten books. It is believed that Riga Veda was composed during the early Vedic period. However, the codification in the book may have been undertaken after its composition in oral form.
Sama Veda
Some of the hymns contained in Sama Veda are taken from Riga Veda. The purpose of doing the same is to set the hymns for chanting by priests.
You can read Sama Veda here
Yajur Veda
Yajur Veda contains rituals to be performed by priests during Yajna. A
Atharva Veda
Among other things, Atharva Veda explains the rituals.