Like many kinds of historical document, account books might at first sight be taken for handy repositories of unvarnished facts. There is, on the face of it, a good fit between the purpose for which the records were created — to provide a reliable record of income and expenditure — and the needs of a certain kind of historian, searching for data relating to prices, living standards, the development of markets and the movement of commodities. Dedicated to the piecemeal itemization of dates, places, purchases and prices, account books are utilitarian documents that seem to have no ulterior motives or hidden designs to stand in the way of modern data-mining operations, whether large or small in scale.