The John Porteous letter book contains 83 letters written by Porteous and 4 by his secretary on his behalf, comprising a total of 251 pages of material. Covering the period between March 8, 1767, and November 1, 1769, the letters primarily concern the business affairs of Porteous and his trading firm, Duncan, Sterling & Porteous, located in New York, Michigan, and Montreal. The letters include numerous details of trading activities; travel between Schenectady, Niagara, Detroit, Fort Michilimackinac, and Montreal; relationships and transactions with clients, traders, and Native Americans; and occasional social and family matters. Recipients of letters included James Sterling (22), John Duncan (10), Robert and Alexander Ellice (13), and James Phyn (5).
The letters touch on numerous details of the trade services provided by Duncan, Sterling & Porteous. In his correspondence, Porteous enumerated and discussed shipments of the various items distributed by the company, such as bear and beaver pelts, spirits, salt, clothing, stones, and food items. Several letters also communicate orders for supplies to Porteous' associates. Porteous frequently noted the sale of items to British soldiers, including Lieutenant Perkins Magra, a cartographer and later consul to Tunis, and Captain Patrick Sinclair, who oversaw the construction of Fort Mackinac. On March 15, 1767, he noted that payments owed to him by the 17th Regiment of Foot were in arrears; elsewhere, he recorded the comings and goings of several military officials, including General Thomas Gage (May 26, 1768). Additional letters discuss the quality and prices of the available products and the increasing difficulty of finding labor at a moderate price.
Porteous's correspondence also sheds light on the firm's efforts to serve their many clients. On June 9, 1767, Porteous wrote to Sterling that he was "at a loss how to excuse" him after a client refused to accept a shipment a brandy that had been sent by Sterling in lieu of rum. Another letter provides a glimpse of the difficulty of working with independent-minded trappers, including one whom Porteous found "unwilling to come to any written agrement[.] only says I may depend upon him" (July 16, 1767). On June 6, 1767, he expressed concern that white traders were slow to arrive at Fort Michilimackinac. Coaxing payments from debtors also proved difficult and is the subject of several letters. Porteous commented occasionally on his encounters with Native Americans and their attitudes toward the British. On July 2, 1767, he reported that Jean-Baptiste Marcotte, a trader near Michilimackinac, had been "Pillaged by the Indians," and in other letters he mentioned gifts intended for the Ojibwe. While in Detroit on February 26, 1768, he assessed Native Americans there as "not very well intent'd this spring" but predicted that no war would take place.
Another frequent topic is Porteous' continual travel between various regions, including New York, Michigan, and Montreal. His journeys were sometimes hindered by the elements, as on March 23, 1767, when he noted to fellow merchant Hayman Levy that he was "obliged to remain" in Schenectady "until our river is open." The letters also contain occasional personal news, such as the health and death of family members and references to pastimes.