The New York Public Library says it received in the mail an overdue library book — a 1926 sex manual — checked out more than 50 years ago.

"An overdue book is nothing to be ashamed of," Billy Parrott, managing librarian at the NYPL's mid-Manhattan branch, wrote in a blog post earlier this week recounting the unusual return. "Let's face it, accidents happen and sometimes books are returned past their due date. It's a small mistake that should not keep you away from the Library."

In this case, it might have had something to do with the subject matter.

The book, "Ideal Marriage: Its Physiology and Technique," by Dutch gynecologist Theodoor Hendrik van de Velde, was returned last year with an apologetic note.

"We found this book amongst my late brother-in-law's things," the note read. "Funny thing is the book didn't support his efforts with his first (and only) marriage ... it failed! No wonder he hid the book! So sorry!"

The note, sent from a Scottsdale, Arizona, address, was signed by a "shocked in-law."

This classic work "concentrates on the cultivation of the technique of eroticism as an art in marriage," the description of its 1980 edition reads. "It sets the sexual relationship in the nostalgic prose of a more leisured age."

"This is not a prude’s book," Frederica Mathewes-Green wrote in the National Review. "Young couples who grab a used copy off the Internet may have even as much fun with it as their great-grandparents did."

The book's due date was Aug. 17, 1959. The library waived the fines.

"Why is society so stigmatized about returning library books late?" Parrott pondered. "Is it a fear of disapproval? The guilty feelings brought about by abusing a free resource and depriving others of those resources?"

Still, the lurid nature of the 323-page book — which he described as "a very wordy and very scientific instruction manual for sexual activity" — had Parrott curious.

"As a librarian, I instantly searched the book for marginalia, wondering if patrons of years past might have taken the liberty to highlight their favorite passages or add notes," he wrote.

But there was only one: an underlined sentence about men who "only care to relieve their own tensions and care nothing for their wives as an individual or mate."