A lost library book about airplanes has been returned to its rightful owner in the Garden State. The tome’s whereabouts remained a mystery for over four decades.

The Mercer County Library System in Lawrenceville, New Jersey, notified its social media followers about the unexpected return earlier this month.

Photos uploaded by the library system show the returned book was a plain blue hardcover entitled "Back to Basics," which was written by the 1977 editors of Flying Magazine, and was published by the Van Nostrand Reinhold Co.

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"This book finally found its way back to the library today after 44 years," the Mercer County Library System wrote on Facebook and Twitter.

The library went on to note that the person who originally checked out the book was able to keep the checkout card "intact" and "looking like new."

The blue checkout card, which stated the library book was due on Wednesday, July 25, 1979, disclosed that losing the card would incur a 10-cent fee.

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Andrea Merrick, manager of the Mercer County Library System's Hopewell Branch in Pennington, New Jersey, told Fox News Digital that the "Back to Basics" book was found in a donation box that was left at the Hopewell Branch.

"Most likely, the book was in someone's home," Merrick said. "We don't have a record of which patron donated the box of books, nor who originally checked the book out."

She continued, "I don't know the exact checkout date, but it was due on July 25, 1979. Using our current circulation period of three weeks, it would have been checked out on July 3. The current overdue fine for books is $0.10 per day, with a maximum fine of $6.00."

The Mercer County Library System's Ewing Branch in Trenton, New Jersey

The Mercer County Library System's Ewing Branch, where the "Back to Basics" book was supposed to be returned, is located at 61 Scotch Road in Ewing Township, New Jersey. Ewing Township is located within the city of Trenton. (Google Maps)

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The Mercer County Library System’s social media posts say the "Back to Basics" book was returned on Monday, April 3, 2023, meaning the book was returned 15,958 days after its due date.

The "Back to Basics" book, which dives into aircraft construction, cockpit mechanic and flight procedures, was checked out from the Mercer County Library’s Ewing Headquarters Branch in Trenton, New Jersey.

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Current online listings for Flying Magazine’s "Back to Basics" book on ThriftBooks.com and eBay have priced the introductory plane mechanics book at around $6.

Flying Magazine was established in 1927 and the niche trade publication continues to cover aviation news and airplane reviews.

Van Nostrand Reinhold Co. was acquired by John Wiley & Sons, Inc. (AKA Wiley), an American multinational publishing company, in 1997, according to Berkery Noyes, a New York City-based independent investment bank focused on mergers and acquisitions.

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A hand returns a borrowed item at library book drop box.

Returning library books after the set due date can sometimes incur late fees. Policies vary at each library. (iStock)

The Mercer County Library System was established in 1928 after locals voted in support of starting a county library the previous fall, according to the library system’s history-focused about page.