"An Infestation of Unicorns" has the pattern of a fairy tale. Its tone, its setting, and its unicorns are all the stuff of fairy tales. From the golden apples (reminiscent of Ancient Greek tales) to the parade of heroes, elements of the fairy-tale tradition abound. In fairy tales like "An Infestation of Unicorns," in which seemingly intractable problems are solved by clever protagonists, the main characters usually gain the security of wealth, or the healing of ills, or the satisfaction of putting stuck-up people in their place. In "An Infestation of Unicorns," an abbot learns the worth of a woman's work and the need for a female pres ence in all-male preserves is emphasized, both issues more associated with modern feminism than medieval folk tales. Yet, Yolen does not stray as far from her folk-tale inspirations as she might seem to do from a modern reader's...