I don't know the Avestan language but I have read the Yasna, about two decades ago, the revealed book of the Zoroastrians.
Yasna presents a monotheistic God named Ahura Mazda.
The texts of the Avesta which are all in the Avestan language were composed over the course of several hundred years. The most important portion, the Gathas, in older (before the works of Johanna Narten 'Gathic') Avestan, are the hymns thought to have been composed by Zoroaster himself. The liturgical texts of the Yasna, which includes the Gathas, is in Older Avestan, with short, later additions in Younger Avestan. The Yasnas' oldest portions may be older than the Gathas, later adapted to more closely follow the doctrine of Zoroaster. Avesta - Wikipedia, the free encyclopedia