Letterbook, 1820-1833, another volume containing copies of outgoing correspondence written by Forsyth as Indian agent. For the years 1820-1827, many of the letters duplicate ones also entered in 4 T, but the later manuscripts, 1828-1833, comprise more than half of this volume.

The majority of the letters for 1828-1830 were addressed to William Clark; these chronicle not only the routine affairs of Forsyth's agency but also the rising tide of discontent among portions of the Sauk and Fox tribes due to their removal from Illinois, white encroachments on mineral lands still owned by the Indians, and white demands for still more treaties, i.e. more land cessions. Forsyth wrote (1830) sketches of more than a dozen leading chiefs and braves of the Sauk and Fox nations for Thomas L. McKenney, but did not include Black Hawk, although the latter figures prominently in contemporary correspondence.

To Lewis Cass, Forsyth sent (1831) an essay giving his observations on the fur trade radiating from St. Louis north up the Mississippi Valley, southwest to Santa Fe, and northwest to the Rocky Mountains and his suggestions for securing better qualified men as Indian agents. Letters in 1832 and 1833 pertain to Forsyth's removal in 1830, to his still-pending claims for 1812 damages, and to a few other events during his years as agent.

Among his other correspondents in the 1828-1833 period were numerous agents, subagents, and other government officials involved in Indian affairs, such as William H. Ashley, John H. Eaton, and William B. Lewis in Washington, D.C., John Green at Fort Armstrong, Andrew S. Hughes in Iowa, George Vashon in Kansas, and W. Warner in Galena, Illinois.