Yazidi religious knowledge is transmitted orally, in qewls, sacred hymns in Kurmanji held and recited by specialists, and for most of the tradition's history it was deliberately not written down. That is the first and most important fact about its attestation.
Two short books circulated in the early twentieth century as Yazidi scriptures, and their authenticity has been seriously questioned since. The manuscripts appeared through intermediaries during a period of intense outside interest, and the scholarly consensus treats them with considerable caution, though the material they contain is not simply invented and its relationship to genuine tradition is still discussed.
The result is that older Western writing about the Yazidis is often built on a foundation the tradition itself does not recognise. That writing also carried, and amplified, the identification of Tawusi Melek with a figure of evil, which is the single most consequential misrepresentation in the entire literature about this religion.
Serious study of the qewls, including work by Yazidi scholars, began in earnest in the later twentieth century and is the appropriate basis for anything said about the tradition now.
The community's recent history bears directly on the record. The attacks and enslavement carried out against Yazidis in Iraq from 2014 caused displacement on a scale that threatens oral transmission itself, and documentation has since become a matter of survival rather than of scholarship.