The primary corpus of Yoruba religion is not a book. It is the Ifa literary corpus, an immense body of verse organised under two hundred and fifty six figures, held in the memory of initiated diviners and recited in the course of consultation. It contains narrative, precedent, prescription and history, and it is transmitted person to person under obligations of training and secrecy.
That has two consequences for anyone reading about the tradition. The first is that the corpus is genuinely vast and genuinely variable: what a diviner in one town holds under a given figure is not identical to what a diviner elsewhere holds under it, and there is no authority that harmonises them. The second is that the parts of it which have been printed were printed by particular people, for particular purposes, and their appearance in a book is not evidence that they were ever meant to be public.
Outside the corpus, the written record begins with missionaries, colonial administrators and, from the late nineteenth century, Yoruba scholars writing about their own tradition. That last group changed the field, and the modern academic study of Yoruba religion is substantially continuous with it.
The tradition also travelled. Orisha religion crossed the Atlantic under slavery and continues in Cuba, Brazil, Trinidad and the United States, in forms with their own histories and their own custodians. Material from those branches is not automatically evidence for West African practice, and the atlas keeps the branches distinct.