6th-4th Centuries B.C.

Achaemenid Inscriptions
The Foreign Trade of Armenia in the Period of the Ancient Persian Empire by H. A. Manandian.

3rd Century B.C.-3rd Century A.D.

Online Texts Concerning Parthia.
The Development of Trade and Cities in the Reign of Tigran the Second by H. A. Manandian.

3rd-7th Centuries A.D.

Karnamak: The Deeds of Ardashir First Sasanian King (224-240).
Kartir's Inscription on the Kabah of Zoroaster.
Kartir's Inscription at Naqsh-i Rajab.

Trade and Trade Centers in Armenia in the Roman-Parthian and the Byzantine-Sasanian Period by H. A. Manandian.

Sassanid Empire Wikipedia article. Excellent summary and links.

Mythology

Iranian Mythology by Albert J. Carnoy.
Eastern Asia Minor and the Caucasus in Ancient Mythologies (Indo-Iranian Mythology section) by Robert Bedrosian.

Art History

Metropolitan Museum of Art (N.Y.) Heilbrunn Timeline of Art History Iran 8000-2000 B.C.
Metropolitan Museum of Art (N.Y.) Heilbrunn Timeline of Art History Iran 2000-1000 B.C.
Metropolitan Museum of Art (N.Y.) Heilbrunn Timeline of Art History Iran 1000 B.C.- 1 A.D.
Metropolitan Museum of Art (N.Y.) Heilbrunn Timeline of Art History Iran 1 - 500 A.D.

Ancient Iranian Civilization and Iranian Peoples

The Zoroastrian Religion in Armenia by Mardiros Ananikian, Encyclopedia of Religion and Ethics, vol. 1 (1908), pp. 794-802

Articles and Essays . Circle of Ancient Iranian Studies (CAIS) website.
Persia, at Livius.org.
Iranian Texts, Sacred and secular at Avesta.org.
Site Index for Parthia.com. Excellent primary and secondary sources.
Encyclopaedia Iranica, Search facility.