Giving it a 4 just on illustrations alone. I love Klassen's dark sense of humor in his picture books, especially when interwoven with vague storytelling. This book is spooky, dark, but never outright disturbing, unless you have a pathological fear of bones. 

I have to admit, seeing a headless skeleton in a darkened doorway did give me the shivers for a split second. 

The book just oozes atmosphere, with all the shadows and rough textures. It's simple, but effective, and it'll no doubt entertain the more morbid children that enjoy the more gothic side of things.

Also, a dark theory popped into my head that Otilla knew that the skeleton was the body of the skull and, to ensure that she will always have a place at the castle due to its dependence on her, she destroys the skeleton so that they can never reunite. She just looks so methodical as she hacks the skeleton to pieces, burns it to ash, and throws it into the bottomless pit. The theory makes Otilla's running away at the beginning a little darker if the theory is sound enough. Is she running away after performing another malevolent deed? Were the people in the town finally ready to capture her after a long string of crime? But the fun is in the mystery after all.