Harlean Hoffman Vision found a rare, limited edition copy of The Picture of Dorian Gray marked with a Chicago Public Library stamp in her late mother's possessions.
Reuters reports that Vision only returned the book after receving assurances that she wouldn't be "arrested".
The library was in the middle of a rare amnesty period, and Vision took the opportunity to return the book unpunished.
Ruth Ledncer, the library's marketing director, said: "She kept saying, 'You're not going to arrest me?' and we said, 'No, we're so happy you brought it back'."
Vision was later informed that the library caps late fees at $10 - if the cap and amnesty were not in place, however, the return of the book would have cost her $6,000 ($3,781).