The text is a samhita, a collection: 1,028 hymns totalling roughly 10,600 verses, grouped into ten books called mandalas. Books two to seven are the family books, each traditionally ascribed to a priestly lineage, Gritsamada, Vishvamitra, Vamadeva, Atri, Bharadvaja, Vasishtha, and their internal consistency of style and metre marks them as the oldest core. Book nine is different in kind: every hymn in it addresses Soma in the act of being pressed and filtered. Books one and ten sit at the edges and are, on linguistic grounds, the latest material in the collection.
Indra takes the largest share, some 250 hymns, a god of storm, intoxication and the slaying of the serpent Vritra. Agni takes around 200, and stands first in the collection because fire is the mouth by which offerings reach the gods. The theology is not systematic. Hymns address gods as supreme in turn, a habit Max Muller labelled henotheism.
Two of the most cited hymns are late. The Nasadiya Sukta at 10.129 asks how the world came to be and closes by suggesting that even the overseer in the highest heaven may not know. The Purusha Sukta at 10.90 describes the cosmos made from the dismembered body of a primal man, and gives the four social orders their oldest scriptural warrant, a passage that has carried enormous weight in later Indian argument about caste, precisely because it is scriptural and precisely because it is a single verse.
The Rig Veda proper is only the innermost layer. Around it accreted Brahmanas explaining the ritual, Aranyakas for forest recitation, and Upanishads. All of these count as shruti. The Rig Veda survives complete in a single recension, the Shakala, out of a number the tradition remembers having.