Denkard
The Dēnkard or Dēnkart (Middle Persian: "Acts of Religion") is a 10th century compendium of the Mazdaen Zoroastrian beliefs and customs. The Denkard is to a great extent an "Encyclopedia of Mazdaism" and is a most valuable source of information on the religion. The Denkard is not itself considered scripture.
Read more about Denkard: Name, Date and Authorship, Structure and Content