The bulk of the collection consists of typescript and photostatic copies of letters and diaries, although some original letters are included. The collection contains the correspondence of Henry H. Spalding (1835-1874), Eliza Spalding (1846-1849), and Rachel Spalding (1870-1874); and diaries of Henry H. Spalding (1838-1843) and Eliza Hart Spalding (1836-1840) describing the overland journey and operation of the mission. The collection's correspondence concerns missionary activities, conflicts between Indigenous peoples and Euro-American emigrants, conditions in the Oregon Country and Walla Walla area, Catholic missionaries, the Whitman killings, and Henry H. Spalding's 1870 trip to the East Coast. Correspondents include Dudley Allen, David Greene, and members of the Prentiss family. Some letters include anti-Catholic sentiments.

Eliza Hart Spalding's diary (1836-1840) describes the Spalding party's overland journey from Plattsburg, New York (February 1, 1836) to Fort Walla Walla (September 3, 1836) and the formation of the Whitman mission; travels from Cincinnati to Walla Walla with Marcus Whitman and Narcissa Whitman; and a daily listing of Bible passages selected for meditation.

Additional materials in the collection include a report on the number and condition of Indigenous tribes west of the Cascade Mountains; lectures and essays concerning the history of relations with the Nimiipuu (Nez Percé) people, missionary labors, and Spalding's views on the impact of early missions in the Pacific Northwest; and books published by the Spalding Mission in Nimipuutímt (Nez Percé) and Npoqínišcn (Spokane) languages.