I need to caution you that Stiles does NOT go into a discussion of “Custer’s Last Stand” but discusses the rest of his life and is good on his fighting the Indians prior to the Little Big Horn battle. He closes with an examination of the court of inquiry over Reno and Benteen’s action that happened several years later. He recommends several books to the reader for accounts of that LBH battle.
He shows how Custer was very competent as cavalry commander during the Civil War and prior to the LBH. He does not denigrate Custer about LBH but makes sure that the reader knows the role Custer and the 7th Cavalry played as 1/3d of the forces being sent against the alliance of tribes that were gathering in the Montana territory.
Custer demonstrated a clear, even insightful understanding of cavalry tactics during the Civil War, on a par with Nathan Bedford Forrest and even James "JEB" Stuart. It was not his fault that his troops were poorly trained and that Union commanders (with the exception of Phil Sheridan) had no idea how to make use of cavalry.
I have a theory that the LBH disaster arose out of a lot of complex history, including Custer's supposed sacrifice of Benteen's friend Maj. Joel Elliot at the Washita. Reno too had his reasons for sabotaging his commanding officer.
But those are topics for another day ...