We have some copies of royal correspondence found in Egypt, written on cuneiform tablets. They used them for messages to foreign kings because they're compact and durable. A lot of the surviving tablets are begging letters asking the Egyptians for gold. We even have some practice tablets from teaching Egyptian scribes to do this.

Anyway I think some of these documents are of communications with Hittite kings, so it would be great to get some instances of the "other side" of some of these letters.