We have no literary sources from that culture, it all got lost in the process of Alexander breezing through, the Seleucids wrecking the place, the Parthians taking it back, the Sasanids knocking over the Parthians and the Arab Muslims rolling in and disposing of much of Iran's traditional culture.

What we do have is inscriptions and artworks and artefacts, though we have more from the Sasanids as far as I know. But for a narrative of anything, only literary sources really give us the detail we need, and as we lack such things from the opposite point of view we unfortunately have to rely on the Greeks, and do our best to read between the lines.