United States Sanitary Commission

And speaking of the Civil War, a contributor to the Association of Professional Genealogists list APG-L points to an unusual record held at the New York Public Library (PDF) with plenty of Midwestern relevance: the records of the Washington Hospital Directory Archives of the United States Sanitary Commission, including more than 9,000 files documenting friends' and families' inquiries about the condition and location of injured soldiers, and the Commission's replies.

According to the collection's scope and content note (PDF),

"The records of the Washington Hospital Directory Archives, particularly the Letters of Inquiry, are rich in content, and will support research in genealogy, military history, medical history, social welfare history, political history, and studies in race, gender, class, ethnicity, and religious culture. The records also illustrate patterns of emigration, migration, language and communication. Inquiries were sent by men, women and even older children from all stations of life," and may amplify or correct other records on the soldiers concerned.

NYPL has an database of these soldiers on line, but the files themselves are not digitized.