Thirty-five years ago, the London-born artist David Gentleman bought a rural weekend home for his family in a village in East Anglia. The appeal of the area, he writes in his new book, In the Country (Full Circle Editions), "was and is its unre markableness…there's nothing much except the pub to attract visitors." But Gentleman's exuberant watercolors, many painted in the open air over the past few years, show that this seemingly ordinary corner of England—with its thatched cottages, medieval church, and stately 18th-century home where the annual fair is held, complete with falconry demonstrations and a sheepdog-herding contest—is rich in natural beauty and enduring tradition. Over a six-decade career, Gentleman, now 84, has illustrated postage stamps, Penguin paperbacks, antiwar posters, and a series of sketchbooks that document his travels…