The best book about Custer is Evan S. Connell’s Son of the Morning Star: Custer and The Little Bighorn
“Son of the Morning Star makes good reading—its prose is elegant, its tone the voice of dry wit, its meandering narrative skillfully crafted. Mr. Connell is above all a storyteller, and the story he tells is vastly more complicated than who did what to whom on June 25, 1876.” Page Stegner
This book is generally considered one of the half-dozen best written about the American west. (And I intend to go read it again when I finish this.)
The best book attempting to tell the vastly more important Indian side of the story is James Welch’s Killing Custer: The Battle of Little Bighorn and the Fate of the Plains Indians.