Hello Dennis -
Dee Alexander Brown (1963) "The Galvanized Yankees," is the story of the several regiments (six? I believe) of "U.S. Volunteer" Infantry regiments, and other units (like a battalion of 11th Ohio Cavalry) recruited from among Confederate prisoners. Brown wrote a pretty comprehensive account that has stood up fairly well, but the type is small and the book is a long, long read. The Federal service of these former Confederates was in the West -- Nebraska, Kansas, Colorado, Dakota Territory, Wyoming, even Utah I believe, and much was "post-Civil War" (later 1865 to earlier 1866). Fascinating history, perhaps more "History of the West" genre than "Civil War History." Apologies if you all know this already.
John