Stunned librarians in Dinnington, South Yorkshire, could not believe it when they received a package containing a Penguin first edition copy of Quatermass and the Pit, by Nigel Kneale, which was borrowed in September 1965.

There was no clue as to who sent the parcel but the long-term borrower need not have worried about the fine as Rotherham Council has a policy of limiting all penalties to £6 a book. The library service currently charges 15p a day for overdue books so, if there was no £6 limit, the borrower would have been liable to a fine of around £2,500 at today's rates.

The package was opened at Dinnington Library by principal library assistant Alison Lawrie, who said: "I thought at first it was just a normal return, until I saw the colour of the pages, they were very brown around the edges. Then I noticed it was an early Penguin book. It was a real surprise, even more so when I looked inside to see the date of return. It's true that some people like to take their time with a good book but 45 years is an incredible amount of time."

Staff think the book was borrowed from the old Dinnington Library, which opened in 1936 and is just a stone's throw from the new library, which opened in 2000.

Ms Lawrie said: "It still has the original paperwork inside. This includes the label with the library name on it along with the five rules for borrowing books and the date label. It was due back on October 15, the length of borrowing at the time was 21 days, so it would have been taken out on loan on September 24, 1965."

She said the identity of the borrower remains a mystery and the only clue is that it was posted from somewhere in Sheffield. Library records do not go back to the 1960s and the paper records from that time were destroyed during computerisation.

Ms Lawrie said: "The person who posted it back to us would not be in any trouble whatsoever. In fact I would really like to know where the book has been living all those years - in a loft or garage, in someone's bedroom or in storage. They've obviously taken care of it. Other than the natural browning on the pages, it's in unbelievable condition.

"There was no letter or note with the book, so obviously the person didn't want us to know who they were. It's a fantastic mystery in itself and has become a real talking point for visitors to the library.

"We have no idea why they decided to return the book after so long but it may be that the person who originally borrowed it has passed away and the family may have found it when they emptied the house, someone may have been tidying up their loft and come across it, or perhaps they thought we would just enjoy the mystery."