Book Search!

A reader writes:



Hello.  I am on a quest to remember a book I read a long time ago.  I THINK the book involved twins.  One part I remember clearly, the girl or girls were giving tours in an old house.  They accidentally found a loose panel in a stair railing.  In it was a letter written by a man that was in love with the then owner of the house.  She was told about the letter and realized her love from long ago hadn't deserted her, he was waiting for her.  It was a touching story in a great book AND I can't remember the title!  Can anyone help me?