The Amarna letters are famous because nowhere else in bronze age history do we have a cache of documents quite like it. Thirteen letters from Kadashman-Enlil, and later his son Burna-Buriash, addressed to Pharoah Akhenaten of Egypt, give us insights into the nuts and bolts of bronze age international diplomacy that even most bronze age folks would have not understood unless they were part of the royal courts. What did ancient kings know about their neighbors, how did they think about their kingdom, and how did they solve disputes? All these and more are in the Amarna letters, and as an added benefit, these letters will take us through the reigns of these two kings, continuing the tale of Kassite Babylonian history.
Also, having now heard so much about these Amarna letters, where can you read them? Well, there is a book with all of them in there called the El-Amarna Correspondence, put out by a group called Handbook of Oriental Studies, whose acronym is HdO because they are German. The book, thankfully, is in English. You can find translations of scattered letters at various places online, but oddly, given how famous this archive is, I am not aware of a good online source for them.