No less a poet than the great Seamus Heaney said of Norman Shapiro's Fifty Fables of La Fontaine, ''It is a pleasure to open a book as sure and sly as these translations. . . . He gets the tune right and the tone right, and manages to echo both the folk wisdom and the poker-faced formality of the originals.''As surely as La Fontaine followed Aesop, Shapiro has now made fabulous fifty more fables of the wonderful La Fontaine--among them ''The Hare and the Tortoise,'' ''The Old Man and the Ass,'' and ''The Frogs Who Asked for a King.'' David Schorr has just as captivatingly illustrated them.