Bookstore sales, which were flat in April, slipped into negative numbers in May. According to the U.S. Census Bureau, bookstore sales fell 4.2% in May, to $1.09 billion, while sales for the entire retail sector rose 6.8%. For the first five months of the year, bookstore sales were up 2.1%, to $6.47 billion, well below the 8.5% increase posted by all retailers.