This is interesting: some of the only remaining pre-Islamic literature found in Iran comes in the form of inscriptions found at various palaces– columns, cliff faces, gates, etc. One reason for the relative dearth of texts is the destructive and acquisitive zealousness of Alexander the Great, who is said to have burned 20,000 cowhides containing the Avesta– a collection of sacred Zoroastrian texts. Here’s an example of a surviving text, the “harem inscription”, taken from http://www.livius.org/ :
[1-8] A great god is Ahuramazda, who created this earth, who created heaven, who created man, who created happiness for man, who made Xerxes king, one king of many kings, commander of many commanders.[8-15] I am Xerxes, the great king, the king of kings, the king of all countries and many men, the king in this great earth far and wide.
[15-27] King Xerxes says: My father was Darius; Darius’ father was named Hystaspes; Hystaspes’ father was named Arsames. Both Hystaspes and Arsames were living at the time -thus was Ahuramazda’s desire- when my father Darius was made king of this earth. When Darius became king, he did much that was excellent. [King] Xerxes says …
[28-43] Darius had other sons, but -thus was Ahuramazda’s desire- my father Darius made me the greatest [mathišta] after himself. When my father Darius went away from the throne, by the grace of Ahuramazda I became king on my father’s throne. When I became king, I did much that was excellent. What had been built by my father, I protected, and other I added other buildings. What I built, and what my father built, all that by the grace of Ahuramazda we built.[44-48] King Xerxes says: May Ahuramazda protect me, my kingdom, and what was done by my father. May Ahuramazda protect this.