The most extensive part of the Cushing Eells Collection contains a series of correspondence Eells kept with his family, especially his two sons Edwin and Myron, throughout the journeys he made to organize various churches and schools. There are also many letters addressed to A.J. Anderson, the first president of the Whitman College. Eells also wrote to some of his friends and collaborators, such as S. B. Treat, Henry H. Spalding, and George H. Atkinson.

There also is a complete series of his diary together with a transcript of it. Other important series are his original lectures, family and personal papers, among which are financial reports and accounts, his diplomas, certificates, and wills as well as clippings and familial accounts about his life. The mission series includes passports (mostly copies) for crossing Indian country, and the few photographs show his family as well as more recent photographs from memorial sites in his honor. Miscellaneous items include newspaper clippings, fragments of letters and notes, legal documents and some published materials.