No Inca account of Inca religion exists, because the Inca had no writing in the sense that word usually carries. They kept khipu, assemblages of knotted cords, and these certainly recorded numbers: censuses, tribute, and accounting and calendrical data, read by trained specialists. Whether any khipu encoded language, and could therefore have carried narrative or ritual text, is an open research question and not an established fact. No khipu has been read as connected prose, and claims that particular surviving cords record histories or myths should be treated as proposals rather than findings.
What exists instead is Spanish. Soldiers and officials wrote first: Cieza de Leon; Juan de Betanzos, who married into an Inca family; and Polo de Ondegardo, an administrator who located and removed the preserved royal bodies and whose reports were written to make suppressing Andean religion easier. Missionaries followed, and the seventeenth-century campaigns known as the extirpation of idolatry generated trial records and handbooks describing local practice in unusual detail, precisely because their purpose was to identify it and end it. Bernabe Cobo, writing in the mid-seventeenth century, preserved earlier material now lost.
Two authors of indigenous or mixed descent stand somewhat apart. Garcilaso de la Vega, son of a Spanish captain and an Inca noblewoman, published an account in 1609 that is invaluable and openly apologetic, presenting Inca religion as an orderly near-monotheism awaiting Christianity. The long illustrated letter that Guaman Poma de Ayala completed around 1615 records detail available nowhere else and was written as an indictment of colonial rule.
The consequences are structural. These sources describe the state cult far better than provincial practice, impose European categories of god, idol and devil, and were often compiled from testimony given under duress. Reconstruction is possible, but nearly every statement about Inca religion carries an implicit qualification: according to Spanish sources, written after the conquest.