I'm just reading Iris Origo's fascinating history, "A Merchant of Prato." The merchant in question, Francesco Datini, kept his entire international trading network together during the late 14th and early 15th centuries with letters he wrote himself. (He apparently didn't trust or wish to pay clerks.) He sent his partners and factors detailed, day-by-day instructions on operating the business, and on their personal conduct, and expected nearly daily reports from them in great detail. The good news is that he saved much of the correspondence, giving historians an incredible record of a 14th-century Tuscan trading house. The bad news is that it took time: he complains in one letter about feeling unwell because he had been writing letters for the last two days straight, without food or sleep.