The unthinkable may be looming for retail book giant Barnes & Noble, and plans for more store closings by year’s end are sending shockwaves through the publishing world.  In the past two years, Barnes & Noble has closed an average of 20 stores each year, although more than 600 remain, spread across all 50 states. Michael Levin, writing in the Contra Costa Times, warned that gift cards from the book chain might be worthless by year’s end. “If anyone gives you a Barnes & Noble gift card, be sure to cash it in by the end of the year. This may be the last year that Barnes & Noble bookstores remain open.” Levin added that publishers are closely watching Barnes & Noble’s struggles. “The publishers have to be running scared,” he wrote. “If B&N suddenly shutters its doors, then billions of dollars of books, which the bookstores take on consignment, go into the limbo of bankruptcy court.” More