Little Big Man
I first read this novel in the flush of the 1960s. It was exciting on just a narrative level (the Wild West! the Custer Massacre! Wild Bill Hickok!); it was deeply informative about the Cheyenne; it was populated with anti-heroes (so fashionable then); it moved me profoundly--especially the scene depicting the death of Old Lodge Skins, the old Cheyenne who had been the moral tutor of our hero, Jack Crabb. I've read it again several times, and it continues to work its magic on me. It accomplishes two of the principal goals of fiction: it entertains; it instructs.