Whose letters are you transcribing? Are you planning to publish?

You may already have seen these books but just in case try the following:

Barnes, Joseph D. Campaigns of the 20th Iowa Infantry with Personal Reminiscences of the War. Edited by M. Lawrence Shannon. Iowa City, Iowa: Camp Pope Publishing, 2016.

Barney, Chester. Recollections of Field Service with the Twentieth Iowa Infantry Volunteers, or, What I saw in the Army: Embracing Accounts of Marches, Battles, Sieges, and Skirmishes, in Missouri, Arkansas, Mississippi, Louisiana, Alabama, Florida, Texas, and along the Northern Border of Mexico. Davenport, Iowa: Gazette Job Rooms, 1865.

Shorey, Henry Augustus. The Story of the Maine Fifteenth: Being a Brief Narrative of the More Important Events in the History of the Fifteenth Maine Regiment. Bridgton, Maine: Press of the Bridgton News, 1890.