Everyone's porcine wonder is back, and just in time for Halloween. Mrs. Watson knows just what Mercy should be for the holiday-a tiara-and-pink-tulle-clad princess. Mercy is dubious, especially after struggling with her dress, until the word "treats" is mentioned. Toast with a great deal of butter is her favorite treat, and that thought is enough to keep our little porker in the game. Following the formula perfected in the earlier titles, the hilarity is found more in the super-saturated illustrations than in the words. Whether Mercy is flying after a terrified cat or snuffling for butter candy, the illustrations are energetic and the sly wit is infectious. One quibble: Because of the placement of spot art, some of the nighttime images show a blue-black darkness and others look like broad daylight. No matter, though-young readers will clamor for more adventures with this irrepressible retro pig and her lovable family. (Fiction. 5-8)