MY MORTAL ENEMY By Willa Cather (Vintage: $8.95) . Willa Cather completed this finely etched miniature portrait shortly before she began "Death Comes to the Archbishop." Myra Henshawe, a worldly, acid-tongued woman, gave up a family fortune to marry for love, but has been only "as happy as most people"; she (and the people in the small town she left) expected this sacrifice to be rewarded with a more singular joy. Henshawe revels in the status and comfort brought by her husband's early success, but when reversals in business reduce the couple to penury, she turns on her adoring husband, blaming him for the bitterness of their poverty. This profoundly disillusioned novel suggests the truth of La Rochefoucauld's maxim: "Love is closer to hatred than it is to friendship."