Staff at Manningtree, Essex were delighted when Great Prime Ministers by John Whittle was handed in by relatives of the borrower who took it out on June 30 1979.
The UK has since had 10 Premiers – and current PM Rishi Sunak was not even born then.
Essex Library Service said: “Staff at Manningtree Library were pleasantly surprised when this title was returned. Despite it being more than four decades overdue, the book was in surprisingly good condition.”
“Our libraries have changed significantly since this item was first borrowed. We now offer everything from e-books and computer facilities to meeting rooms, events and activities.”
Library chiefs decided to waive any fine for the book’s late return.
The maximum penalty is capped at £8.75 per overdue item.