Unprepossessing in appearance and concept but sprinkled with sly visual amusement, this shows an animal children's costume party invaded by a strange guest in a bear costume. As ""It"" for hide-and-seek (when roller-skating ducks ""hide"" tails-up, heads under the rug), the bear makes short work of the game. ""He must have peeked,"" say the sheep. He pins the tail on the wrong donkey (a pained live one) and wins at pie-eating and apple-bobbing. ""He didn't seem to know the rules."" When it's time for unmasking, readers won't be surprised to find a real bear under that zippered bear suit. Nor is the conclusion unexpected: The animals flee in terror, but return to resume the festivities and award him best costume when the bear confesses ""I've never been to a party before. I'm new here. I was lonesome."" But the laconic humor, effected through an interdependence of sparse words and sparse pictures, is right for the easy-reading mode.