The Amarna letters are about 350 baked clay tablets, found in the ruins of Akhetaten. They are mainly letters written to the king of Egypt from the kings and princes of Assyria, Babylonia, Mitanni, Cyprus, Palestine, Syria and Hatti (the Hittites).
The tablets are written in cuneiform script, (wedge-like shapes), and in Akkadian, the diplomatic language of the day. The more powerful kings called the Egyptian king “Brother”, but their letters could get very frosty – in one, the king of Mitanni is furious with the king of Egypt for detaining a Mitanni messenger for six years. Lesser kings addressed the Egyptian king with more respect, calling him “My God”, or “The Great King”. They begged for help, or tried to turn Egypt against their rivals.
Source: The Usbourne Encyclopedia of Ancient Egypt