1-Be wary of online/offline/oral sources given that it is a very old (one of the oldest) religion and most people deform it and interpret it in their own way.
2- Not many credible sources but I will list the ones that are
Zarathustra and Zoroastrianism: A Short Introduction is a scholarly book by Stausberg and is very informative. It dives into who Zarathustra is/was and how he changed the religion.
1- The Avesta
2- Pahlavi Texts (e.g. the Bundahishn, the Denkard, the Book of Arda Viraz Namag, etc.)
3- Royal Inscriptions (Achaemenid Royal Inscriptions)
4- Greek and Romain sources (mythology)
**If I forgot any I am sorry