By most estimates, over 200,000 ethnic Germans served in the Union armies during the Civil War. Dividedly roughly evenly between Democrats and Republicans (even though they undoubtedly had reservations about the influence of nativist Know Nothings in the latter party!), the western German communities tended to be less conservative [with a higher 'forty-eighter' influence] than the large Pennsylvania and New York populations. However, relative to other ethnic or racial groups, Germans are still comparatively neglected in the Civil War literature--although this is slowly changing. Unfortunately, the popular (and highly inaccurate) view of their combat service is probably as goofy 'I fights mit Sigel' types who ran at Chancellorsville and Gettysburg. [By the way, I see there is an upcoming book on the subject, Chancellorsville and the Germans: Nativism, Ethnicity, and the Creation of German-Americaby Christian Keller (Fordham University Press, 2007).]
Much of the more recent work centers on the West. Here is a list of a few available and upcoming studies that might be of interest:
One of the great byproducts of studying the early campaigns in Missouri is the gaining of a proper appreciation for the critical role of Germans in the Trans-Mississippi West. See The Battle of Carthage: Border War in Southwest Missouri, July 5, 1861, Wilson's Creek: The Second Battle of the Civil War and the Men Who Fought It, and Pea Ridge: Civil War Campaign in the West for a window into the military contributions of Germans in the Trans-Mississippi theater, especially their central role in the securing of Missouri for the Union. I've previously written about Robert Owens' 1989 work Hier Snackt Wi Plattdeutsch, which, in addition to a cultural history of Low Germans in Missouri, has a few great chapters on the Civil War, most particularly the 1861 fight at Cole Camp. Finally, the structural changes to the political landscape of Missouri due largely to a large and rapid influx of German immigrants are traced in Louis Gerteis' Civil War St. Louis.
University of Oklahoma Press is also publishing a memoir later on this year (mentioned here earlier) titled Marching with the First Nebraska: A Civil War Diary.
Author Joe Reinhart's growing body of work deals with the Western theater, researching the contributions of Germans in two regiments, the 6th Kentucky and the 32nd Indiana: A History of the 6th Kentucky Volunteer Infanty U.S.: The Boys Who Feared No Noise , Two Germans in the Civil War: The Diary of John Daeuble and the Letters of Gottfried Rentschler, 6th Kentucky Volunteer Infantry , and most recently August Willich's Gallant Dutchmen: Civil War Letters from the 32nd Indiana Infantry. Reinhart was a recent guest on Civil War Talk Radio. I was pleased with the detailed discussion of Rowlett's Station, a subject covered in depth in the 32nd Indiana book. Apparently, Reinhart has also put together the most detailed map of that engagement to date.
University of North Carolina Press published Germans in the Civil War: The Letters They Wrote Home in 2002 as part of their Civil War America series.
Another recently published regimental history is Donald Allendorf's Long Road to Liberty: The Odyssey of a German Regiment in the Yankee Army : the 15th Missouri Volunteer Infantry (Kent State Univ. Press, 2006. Hardcover, maps, notes, bibliography. 364pp). I haven't seen this one yet or read any reviews, but this highly distinguished unit certainly is deserving of a good modern regimental history.