THE MARRIAGE OF CADMUS AND HARMONY By Roberto Calasso. Translated by Tim Parks. Illustrated. 403 pp. New York: Alfred A. Knopf. $25. THERE were no mythology books in fifth-century Athens. Everyone knew the stories. Boys were taught to recite Homer's accounts of Achilles' anger and the return of Odysseus, and other epic poems recounted the myths of the love affairs of the gods and the adventures of the famous heroes. Myths were related to women working at their looms, and male citizens could watch how the dramatists would tell the same stories in new and exciting ways. But after the mid-fourth century, when King Philip of Macedon defeated Athens, and his son Alexander, through his conquests, spread Greek culture throughout the Mediterranean area, the first Greek mythology books were written for the many people who did not know the stories. Perhaps the most influential surviving example of this genre is the epic "Metamorphoses" or "Transformations," by the Roman poet Ovid, which recounts in Latin verse how certain mortals were turned into animals or plants as a result of their encounters with the gods.