The book "The Gray Ghost" by Seckatary Hawkins. Scout loved this book and this is what Atticus reads to here right before she goes to bed at the end of the novel.

Atticus turned the book over. "Something of Jem's. Called 'The Gray Ghost.'"
I was suddenly awake. "Why'd you get that one?"
"Honey, I don't know. Just picked it up. One of the few things I haven't read," he said pointedly.
"Read it out loud, please, Atticus. It's real scary."
Pg. 375