Gettysburg deserter or Civil War hero? Scholar/sleuth Homer Kelly and his wife, Mary, solve the mystery of her great-great-grandfather in this "clever" novel ( Booklist).   Homer and Mary Kelly have wandered through Harvard University's Memorial Hall dozens of times, but never have they lingered over the long list of alumni who died for the Union during the Civil War. One afternoon, the setting sun casts its light on the name of Seth Morgan, Mary's disgraced great-great-grandfather. She knows little of her ancestor's life, for family lore holds that he was a deserter, and a blight on the Morgan name. But as she and her husband dig into the dead man's story, they find something astonishing.   The mystery deepens as the story shifts from past to present. Even in 1863 it was difficult to know just what happened on the blood-soaked fields of Gettysburg, but no matter what it takes, Homer and Mary will find truth, and restore the honor of a man who died fighting for his country.