By mistake, I ruined the library book. What is the penalty?
Talk to the librarian and/or add a note about the damage when you return the book. Librarian will assess the damage and decide on the fines. If the book is usable, there will be just a minimal penalty. If the book is beyond repair, you will be invoiced the price of the book as replacement cost, plus a $10 processing fee. This is done on a case-by-case basis.
Better to call the library ahead of time to discuss the situation.