Barnes & Noble Inc. reported a small decline in earnings for the quarter ended Jan. 30, as customers continued to embrace both electronic books and the convenience of online shopping. Steve Riggio, chief executive, said he was most excited about the retailer's digital business, noting that e-book sales are "exploding" and that the retailer's market share in certain e-book categories "will soon exceed our market share of physical books in those categories." E-books, however, are still a small part of the industry; publishers estimate they generate 3% to 5% of total book sales.