The Swenson Center has some collections of documents from the American Civil War. See for example:
MSS P:55 Mathilda Cassel Peterson Danielsson papers
The papers consists of photocopies of a typescript manuscript prepared by Earl Check of Madrid, Iowa, entitled "Civil War Letters to New Sweden, Iowa," and biographical information about the Cassel family from Kisa parish in Östergötland who settled in New Sweden, Iowa in 1845.
MSS P:109 Warner family papers
The papers include three handwritten letters. One is signed by "Kesti" to unspecified recipient 1862; the second is addressed to Gustaf in Athens, Alabama 1864, and the third is addressed to Mrs. Benjamin Wallin 1942.
MSS P:129 Mercy Brittain papers
The papers include typescript copies of letters in English from civil war soldiers John Brittain and Daniel Cowell to Mercy M. Brittain. The papers also include one handwritten letter in Swedish from C.J Larson addressed to his wife (no name).
MSS P:222 Charles Lindman papers
The papers include a handwritten text entitled "text of Thanksgiving" which was read aloud at the First Lutheran Church, Moline, Illinois on June 15, 1856 for the life of Johanna Forsberg; Deacon's minutes from First Lutheran Church, Moline, Illinois 1850-1860; Diary and notebook kept by Carl Lindman during the Civil War ca. 1862; a copy of the Augustana Synod constitution from 1870; sermon texts; speech by unknown individual; misc. loose-leaf pages.
SA-108 Hans Westerlund
1842-1874-- Papers, Correspondence (including Civil War letters)