That unreturned library book could threaten your credit score if it becomes a permanent fixture on your shelf. According to the New York Times, libraries are treating borrowed materials as debts and siccing debt collectors on borrowers who fail to pay longstanding late fines. The Queens Public Library has used Unique Management Services to collect over $11.4 million from delinquent borrowers, who may not realize that their unreturned books could eventually stand between them and a mortgage.