I did notice a couple of noteable errors in the "background" information regarding American Indians in the war (Chap 3).
The author refers to Chilly McIntosh as "Chitty".
Regarding the Cherokee removals from the southeast over the Trail of Tears, he incorrectly states that there were 8,000 of which half died. There were over 16,000 Eastern Cherokee (not to be confused with the Eastern Band of the Cherokee in NC) who were rounded up into stockades and then moved west over the Trail of Tears. The most commonly repeated death toll is 4,000 though this number is disputed by serious researchers. It can be confirmed that from the time the Eastern Cherokee were rounded up by the US Army through their first year in Indian Territory, well over 4,000 died from disease, exposure, etc., and, as always, the highest mortality among infants and the aged.
On the issue of the first Indian units facing each other in regimental strength, if you put enough qualifiers, you can support any claim. The Indian units at Round Mountain (Nov 19, 1861) and Chusto Talasah (Dec 9, 1861) were certainly "regimental strength" in number. Opothleyahola's "Loyal Creeks" were organized into "units" with a leadership structure but, in spite of that, not typically considered a true military organization. They were certainly not US troops. One can argue either side of the debate as to whether the three battles in Nov-Dec 1861 are "Civil War battles", since no US troops were involved etc. or whether Opothleyahola's warriors constitued a "military organization". Either way, they fought in a organized millitary fashion in the first two battles. At Chustenalah, there may have just been a small "guard" of Loyal Creeks remaining that was quickly overwhelmed by the Arkansans.