SAM WALTON, the fabled founder of Wal-mart, the largest retailer in the world, could be firing a warning shot across the bows of British retailers in the form of his self-promoting book, Made in America. Wal-Mart is sending a load of baseball bats, bunting and other material to boost sales of the paperback, which is being launched this week. But British retailers are getting their defences in order. Harriman House, the publisher, has already sold a couple of copies to Asda, two more to main board directors of Sainsbury, and 20 to B&Q. Do they know something we don't?