THE MYTHS OF MOTHERHOOD by Shari L. Thurer. (Houghton Mifflin: $22.95; 381 pp.) "Good mothering is a cultural invention," writes Shari Thurer, psychologist and mother. And guess who has traditionally invented it? "My aim in writing this book is to make mothers' internal lives acceptable to them, to undo some of the angst among mothers. . . ." Indeed, once we have somersaulted through the misogyny and infanticide of the ancient Greeks, Romans and Phoenicians; the Madonna worship of the Middle Ages; the Holy Mother/impure prostitute duality of Christianity; the beatific Renaissance; the good peasant mother of the Reformation; the Puritan's witches; the 19-Century pedestal for the moral mother; Dickensian sacrifice at the altar of domestic happiness in the Victorian era; and the social and biological revolution (Oh, I'm sorry, did you miss it?) of the 20th Century, the reader feels, well, tired. Very, very tired. The conclusion? The ideology of motherhood is fickle. Don't let cultural imperatives get you down.