Youngsters who have read Sonia and Tim Gidal's My Island in Greece (Pantheon- 1960) will be somewhat confused to encounter the same story with different rimmings. For here again is Petros, pet of Vassili, a boy of Mykonos. And here is the miracle of Petros as a tourist attraction, of his departure for another island -- and of his return. But the way the return is engineered is different in this book; instead of Petros himself establishing the identity of his owners, Vassili uses a trick to lure him back. The story is slight -- one of the penalties of telling a true story -- and the quality of island life would have benefited by more colorful details. Dimitris Davis, the artist, is a resident of Greece, and his sketches, while peasant Greek in feel, convey little of the special quality of this particular island.