The collection consists of a diary and letters of A. B. Harden. The diary details Harden's overland journey to Oregon in 1847, including notations of distances, descriptions of interactions with Indigenous peoples, and mentions of incidents such as the accidental death of an 8-year-old boy in the emigrant party. The diary also covers several months of Harden's life in Oregon, with mentions concerning news of the Whitman killings and the subsequent war between Euro-American emigrants and the Cayuse people.

The letters, addressed to Harden's family, describe life in Oregon and Oregon's climate and people. The letters also discuss developments in the war between Euro-American emigrants and Cayuse people, such as the death of Cornelius Gilliam; list members of the Oregon Provisional Government; and include some poetry by Harden. The collection also includes a photocopy of a letter in which Harden advises his brother, J. D. Harden, to use the southern route when coming to Oregon so as to avoid tolls on the Barlow Road, and on the back of which is a map depicting Astoria and towns in the Willamette Valley.