Be careful with the Cunningham book, it's not very reliable. What he didn't know, he pretty much made up to fill out the narrative. The two best books on Stand Watie are unfortunately out of print, but you might inquire at the library:

Wilfred Knight, RED FOX: STAND WATIE'S CIVIL WAR YEARS IN INDIAN TERRITORY (Glendale, CA, 1988)

Kenny A. Franks, STAND WATIE AND THE AGONY OF THE CHEROKEE NATION (Memphis, TN, 1979)