Doll Bones was not as scary as I thought it would be (thank goodness). If I had to classify it into one category, I think I would say it was more of a coming of age story than a ghost story. The creepy ghost living inside the doll was ghost was secondary.
The one thing that really set Doll Bones apart from other middle grades books that I have read was the believable emotions between characters. Holly Black did a great job showing the budding complexities of preteens. Zach, Poppy, and Alice have differing home situations, and each child has a reason for continuing the quest set before them. They are each at a very delicate time in their lives, where many things are changing and they don't know how to deal with those changes. This book has a great message about growing up and friendship. Any reader will be able to find something to relate to, regardless of gender.
It is also has a quick pace to keep a younger readers' attention. Without a steady supply of mystery and action, a younger reader might get lost before finishing nearly 250 pages. I felt like the various plots merged well together and kept the pacing on track. The ghost story helped fuel the quest and kept the characters engaged, which in turn kept me as a reader turning the pages.
Overall, Doll Bones is best suited for readers ages 10-12 (maybe I bit younger if they can handle the length of the book).