The Lachish ostraca are letters and notes written in ink on potsherds, found in the burnt debris of the city gate at Lachish. They date to the final Babylonian campaign against Judah, and they are the closest thing we have to contemporary correspondence from the events Jeremiah describes.
What they contain
Military routine, mostly: orders acknowledged, movements reported, a subordinate defending himself against a rebuke. That ordinariness is the point. They were not written to be read later.
The most quoted is the letter reporting that the writer is watching for the fire signals of Lachish, because the signals of Azekah can no longer be seen. Jeremiah 34:7 names Lachish and Azekah as the last fortified cities of Judah still holding out. The correspondence is striking and it is not a quotation of one text by the other: it is two sources describing the same military situation.
Where the care is needed
Several of the ostraca are badly damaged, readings are contested, and some of the more dramatic reconstructions offered over the years rest on restored text. Any claim built on a specific phrase has to say whether that phrase is on the sherd or supplied by an editor.
How it is cited here
By Internet Archive identifier, which resolves to the item record and returns the title.