It's been some years since I read it, and I've hardly looked in it since... My general advice would be to stay away from it. Apart from a number of factual errors, a major flaw is the author's lack of knowledge of the broader picture of the war in the Trans-Mississippi. Another serious drawback is his journalistic, melodramatic prose and an uncritical glorification of not only Watie, but all Confederates in the Indian Territory. Not unlike Jay Monaghan's "The Civil War on the Western Border" with a Confederate bias...
Not all parts of Cunningham's book are equally bad though, but I don't care much for it, and I'm a bit surprised that it was reprinted by the University of Oklahoma Press - which would tend to give it a lot more credibility than it deserves.