MY LOVE, MY LOVE OR THE PEASANT GIRL by Rosa Guy (Holt, Rinehart & Winston: $11.95). Rosa Guy's brief 11th novel, set in the lush West Indies, is based on Hans Christian Andersen's fable "The Little Mermaid." Departing from the realism of many of her other novels, here the Trinidad-born writer, a founding member of the Harlem Writers Guild, tells the story of a modern-day black peasant girl's love for a rich mulatto boy from the city. Abandoned at birth by her mother and raised in the mountains by a couple who named her Desiree Dieu-Donne (God-given desire), the peasant girl is moved to pursue the man, Daniel Beauxhomme, by her own passions and those of the volatile and often violent gods who rule the lives of the peasants. Her stay in the city in pursuit of Daniel brings home the truth of her mother's admission: " 'I fear more the revenge of the rich with their police than I fear the revenge of the gods.' " Guy is a first-rate storyteller and an accomplished writer in firm control of the passion of her characters and her material. The only problem with "My Love, My Love" is that Guy's fairy tale now and then reads like one written for children instead of adults, as the charm and simplicity of her writing cross the fine line into sentimentality, and the storybook characters fail occasionally to engage an adult sensibility.