The Incas kept detailed records and did not write. Administration ran on the khipu, knotted cord, which encoded number reliably and whose capacity to encode narrative is still argued over. No Inca account of Inca religion survives in an Inca medium.
Everything therefore arrives through Spanish. The chroniclers wrote within decades of the conquest, from interviews with people who had lived under Inca rule, and they wrote with purposes: to justify the conquest, to argue about how the colony should be governed, or to equip the campaigns against continuing native worship. Cieza de Leon, Betanzos, who married into the Inca elite and worked from her family's testimony, and the later systematic account of Cobo are the principal witnesses, and they disagree on important points including the standing of Viracocha.
Guaman Poma is a different kind of source: an indigenous Andean author writing a vast illustrated letter to the king of Spain, both an insider account and an argument.
The Huarochiri Manuscript is the exception that shows what is missing elsewhere. Compiled around 1600 in Quechua, it records the traditions of one province in that province's own language, and it is the closest thing to Andean religion describing itself.
The campaigns to extirpate idolatry generated their own archive, hostile and detailed, and are a further source for what people were still doing a century after the conquest.