A FRENCH AFFAIR by Michael Kenyon (St. Martin's / Thomas Dunne: $18.95; 210 pp.) Michael Kenyon has done for/to southwest France what Peter Mayle did for/to Provence: exposed it to the light in all its charm, intransigence, joie de vivre and perversity. In other words, Kenyon has taken the valley of the Lot River, the town of Cahors in particular, and spoiled it for the rest of us. (The Brits, brollies furled, already are snatching up, at Chelsea prices, flats whose roof beams "date back to Louis the Quarrelsome and John the posthumous.")
With wife and three daughters, Kenyon moves to Cahors in 1976, leaves, comes back for eight years, leaves, comes back. Prime lure, of course, is the food. The valley is fabled for its foie gras, truffles, cheeses and the hungry Kenyons jam themselves into the local scene like grain into a fat goose. Michael goes on a highly successful truffle hunt ("we park and debouch into scrofulous woodland"). Wife Catherine helps harvest grapes for the distinguished "black wine" of the region--a wine Peter the Great prized as a cure for ulcers (they don't make ulcers the way they used to)--and in three back-breaking weeks gains 22 pounds at the table of the vintner's wife.