The Avesta

This is the Zoroastrian holy book. It is actually split into two sections. The actual Avesta consists of the seventeen Gathas, Zoroastrian hymns which were written by Zarathustra, the founder of Zoroastrianism. The Younger Avesta was composed later and contains commentary on the Avesta and newer stories and content.

You can find this and more Zoroastrian holy scripts here and here. The first link is from an actual Zoroastrian source, and the second source is my go-to source for all religious texts.