Letters of Note has a rather peculiar letter up. It’s written on birch bark and contains information about an arrest by a princess over a slave trade and is dated somewhere around the early 12th century:
Frankly, I wasn’t aware of these documents, but fortunately Wikipedia comes to the rescue:
On July 26, 1951, during excavations in Novgorod, a Soviet expedition led by Artemiy Artsikhovsky found the first Russian birch bark writing in a layer dated to ca. 1400. Since then, more than 1,000 similar documents were discovered in Staraya Russa, Smolensk, Torzhok, Pskov, Tver, Moscow, Ryazan, although Novgorod remains by far the most prolific source of them. In Ukraine, birch bark documents were found in Zvenigorod, Volynia. In Belarus, several documents were unearthed in Vitebsk and Mstislavl.
According to Valentin Yanin and Andrey Zaliznyak, most documents are ordinary letters by various people written in what is considered to be a vernacular dialect. The letters are of a personal or business character. A few documents include elaborate obscenities. Very few documents are written in Old Church Slavonic and only one in Old Norse.
Fantastic! Here’s a page with a few pictures of birch bark letters, in case you want more (and are fluent in whatever vernacular dialect it is they are written in).