Thank you  I did not say it was a single book but what you linked to says it is a corpus and it is often published at once though apparently there is debate whether it was all written by Zoroaster  At least for what Zoroaster wrote I think philosophy does play a part mostly involving semantics difficult knowing esoteric philosophy helps to interpret spirituallydivinely inspired texts as you described but perhaps also with gematria or as HPB wrote perhaps with  other methods than these latter   Since many esoteric mystics have consensus on philosophy not the least of which is ethics methods of interpretation are important  For example such mystics would say animal sacrifice in the Avesta should not be done but is symbolic of something as it is thought to be in the Vedas etc