It turned out to contain a copy of “A Complete Guide to Heraldry” by A.C. Fox-Davies that was checked out in 1986.

“We’re most pleased” that the book came back to the library, said Diane Nichols, associate dean of libraries operations and director of Ekstrom Library.

Not only that, but it returned with a $100 bill tucked inside.

The sender obviously was trying to make up for overdue fines, said Raymond Slaughter, who discovered the package’s hidden secret. So mysterious was the parcel that Slaughter, who works on the loading dock, couldn’t tell what department to send it to without opening it.

This is the first time that he has seen someone put money in a book that’s shipped to the library, said Slaughter, who walked the book and its unexpected contents to Nichols’ office.

Nichols said that, although it is rare, she has seen a few long-lost items come back under similar instances in her career.

The money went to good use: the book replacement fund.

The book? It’s back on the shelf ready for someone else to check it out.