The biography of local poet and writer Edwin Muir by P H Butter was due to be returned to Orkney Library before October 24, 1973.
It was finally checked back in after being found during a house clearance.
Library manager Gary Amos told BBC Scotland: "The person who returned it was amused. There was no fine - it was from too long ago. These things happen periodically."
He added: "It will be looked at carefully, before going back into stock.
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