Watercolors by Diana Sperling with a text by Gordon Mingay. St. Martin’s, $25.00. Diana Sperling belonged to a sizable, moderately prosperous family living in the English countryside with the assistance of farmhands, horses, donkeys, and hospitable neighbors. She was also a watercolorist with a charmingly light, comic style and a quick eye for absurd contretemps, especially one provoked by recalcitrant wildlife. The family mounts, technically domesticated, must be counted as wildlife, along with spiders in the boudoir and the artist’s sister Isabella, who could fall off anything and did. The paintings are a delightful record of what must have been a very likable family. Mr. Mingay’s text explains details that might otherwise mystify a modern observer.