The Secret in the Old Attic by Carolyn Keene, 177 pages
From the MARC: "Nancy Drew must use clues in a dead man's letters to find some unpublished musical manuscripts that will save an old man and his granddaughter from financial ruin."
Nancy Drew was a staple of my childhood reading, and The Secret in the Old Attic is the mystery that most had me in its thrall. It stuck in my imagination to the point I remember illustrating various scenes on my own.
