1983 Edgar Award winner by Cynthia Voigt, author of the Tillerman cycle books.
While the narrator of this little book is just thirteen, the story is appropriate for this age and up. Winner of the Edgar Allen Poe Award, the mystery begins with a young girl sorting family papers for Mr. Thiel. Even though he's a friend of her Aunt Constance, she finds him a little terrifying. Instead, she finds herself drawn to another man whose warmth and friendly manner is more appealing to a young person. As the mystery unfolds, she finds herself the center of it all. To tell more about the story would give too much away. My advice is just to read this one and discover the mystery for yourself.
I loved this book when I read it as a girl. I had to get it from paperback swap again to read it. It was just as good as I remember. I love the mystery and the interesting characters!