This is an awesome Nancy Drew book! It’s not one of the original Carolyn Keene stories but it still is a great mystery. There is also a PC computer game, with the same title made by the company “Herinteractive”
Nancy Drew takes a vacation in Wisconsin when the library of the place she is vacationing at is vandalized. Nancy, along with her friends George Fayne and Bess Marvin, must survive an unknown assailant while discovering the secret passageway’s inside of an old castle.