Ernest Hemingway’s ‘For Whom the Bell Tolls’ tells the story of an American volunteer who leads a small band of rebels, hiding in the mountains behind fascist lines, who have to blow up a bridge. There’s a great section that describes the scenes in one village following the “revolution”. Unpopular landowners are systematically executed in public. This is just one of the many ways Ernest highlights the brutality that marred the era.
For an insight into the fascist’s revenge checkout Stuart Christie’s book Granny Made Me An Anarchist or Giles Tremlett’s ‘Ghosts of Spain: Travels Through a Country's Hidden Past’. Ultimately ‘For Whom the Bell Tolls’ is great literature that happens to be about the Spanish Civil War, rather than a great book about the Spanish Civil War. It’s still well worth a few hours of your life.