A truly remarkable resource here is the set of records compiled by Francisco di Marco Datini, the leading merchant in the town of Prato, north of Florence, between c.1376 and 1410.

They cover all his trading activities (much of his business was with southern France, but he ranged as far as the Black Sea) in great detail, as well as adding a number of series of highly revealing personal correspondence, some with his various agents, but much of it with his wife and his best friend. The archive, in total containing c.500 account books and 50,000 pages of business papers, was left to the city after his death, but was forgotten. The surviving papers were found stuffed into a hessian bag in a neglected cupboard under a stairwell in Datini's old residence in 1870.

A book length English-language account of the archive and its significance can be found in Iris Origo's book The Merchant of Prato (1957). Or there's a short introductory essay here.