Norwich lending library is housed in the Forum building
Private detectives have been hired by a council to crack down on residents who are flouting library rules.
Norfolk County Council said it had spent more than £80,000 in the past three years on debt recovery.
They are reclaiming unpaid school transport fees, locating owners of abandoned vehicles and tracing missing library books, DVDs and CDs.
Their most effective campaign has been collecting unpaid library fines and recovering books, CDs and videos.
Jennifer Holland, head of the council's library and information service, said more than £86,000 worth of stock had been recovered.
"In the vast majority of cases, overdue charges are an effective way of ensuring our books are returned," she said.
"When this doesn't work, we take a number of steps including letters and phone calls from our stock recovery officer.
"As a last resort, where stock is of high value and we have no forwarding address, we will also use external debt recovery agencies and private investigation."