According to their home page, The Arthur H. Clark Company has been publishing western themed titles since 1902. Today, they are an imprint of the University of Oklahoma Press. Several titles from their Frontier Military Series may be of interest to readers of Civil War era books.
The most recent FMS release is last month's Powder River Odyssey: Nelson Cole's Western Campaign of 1865 the Journals of Lyman G. Bennett and Other Eyewitness Accounts by David E. Wagner (March 2009). My review will be posted in the near future.
I've also skimmed loan copies of two backlist titles from the same series covering Indian conflicts during and surrounding the Civil War years. I think both are worth owning someday (especially the first).
* Guarding The Overland Trails: The Eleventh Ohio Cavalry In The Civil War by Robert Huhn Jones (2005). Jones is probably best known to Civil War readers or bibliography trackers from his book The Civil War in the Northwest: Nebraska, Wisconsin, Iowa, Minnesota, and the Dakotas, published way back in 1961.
* Regulars in the Redwoods: The U.S. Army in Northern California 1852-1861 by William F. Strobridge (1994). This study details the army's struggle to maintain peace between the white settlers and the Indians of northern California in the decade preceding the Civil War. A short chapter at the end deals with the series of military operations underway at the time of the firing on Fort Sumter.
The books from the series, at least these three that I've seen, are bound in handsome blue cloth with gold foil stamped spines (and sometimes front covers).