Borders Group Inc., the second-largest U.S. bookstore chain, filed for bankruptcy protection after years of sharp sales declines that made it impossible to manage its crushing debt load, and said it plans to close about 30 percent of its stores. Its inability to garner significant online business and its near absence from the growing digital books market have made it difficult for Borders to compete with larger rival Barnes & Noble Inc. and online retailer Amazon.com Inc. Borders, which has 6,100 full-time employees, operates 508 namesake superstores as well as a chain of smaller Waldenbooks stores.