The sun lights the day and the moon lights the night, and morning seldom breaks to find them both up in the sky. But when the moon does stay up past dawn, the people of Oaxaca, Mexico, have an explanation: "The moon was at a fiesta." Once again, Matthew Gollub and Leovigildo Martinez have created a sumptuous original tale set amid the rich folk culture of Mr. Martinez's own Oaxaca--a tale as exhilarating as a moonlit fiesta full of music, tamales,...