I disagree with the idea of “no harm done”. It is certainly not great harm…but then again, if you were the one waiting for the book, do you want to wait longer than is necessary? What if you need it for a school assignment with a limited time frame?
The issue in the OP title is not about fines and such but whether there is a moral obligation to return the book on time. Clearly the moral obligation exists since, by borrowing the book in accord with library policy, you make the promise to return it on time.
Remember the story of the talents (Mt 25:14-30)…Those who are faithful in small things will be given more. Those who are not faithful in small things will lose what little they have.
Peace
James