The letter book includes retained copies of some 125 pieces of military correspondence written by Swearingen from Pittsburgh, ranging in date from 18 April 1806 to 13 November 1807. There are also a few copies of letters received, receipts, and other documents. All entries appear to be in Swearingen's hand. The volume itself measures 22 cm. and includes 91 leaves, with blanks; it is bound in the original marbled boards, back strip lacking. There is a label with Swearingen's name on the front pastedown.

It is clear from the letters that, at the time of Swearingen's arrival, the army's post in Pittsburgh was ill-equipped: ". . .I have neither n. commd [non-commissioned] officers or privates at this place who are or could be made use of, having at present only one sergeant whose time will expire this fall & four privates two of whom will be discharged between this and that time. . . .The works so much out of repair, that it has almost become as public as the streets in the Town. . . ." (24 May 1806). While some of the letters deal with personal and purely local matters—pay, recruiting, supplies, and so on—many more deal with events further afield. From Swearingen's position at the head of the Ohio, he served to facilitate the passage of men and supplies to army outposts in the West: to Fort Massac on the lower Ohio, to Fort Pickering at Chickasaw Bluffs (present-day Memphis), to St. Louis. He also assisted delegations of Indians travelling to and from Washington. His most frequent eastern correspondents were Secretary of War Henry Dearborn; Artillery Regiment chief Col. Henry Burbeck; William Linnard, U.S. military agent at Philadelphia; Caleb Swan, the army's paymaster general; and William Simmons, the War Department accountant. Other letters are to army officers in command of the service's far-flung outposts. Perhaps the most unusual letters are a series written to Dearborn and Burbeck in December 1806, providing intelligence on groups of men rendezvousing in western Pennsylvania to join "the supposed expedition of Col. Burr". These deal directly and at some length with the Burr conspiracy: former Vice President Aaron Burr's Western schemes that led to his arrest on charges of treason in 1807.