Fear of the dark is fear of the unknown. If you are unable to see what is out there, your imagination is quite adept at filling in the frightening gaps for you.
The Dark, by Lemony Snicket and illustrated by Jon Klassen, focuses on Laszlo, a young boy who tries to preempt the dark from visiting his room at night by meeting it where it lives during the day, down in the basement. Poor Laszlo finds that his journey does little good after his nightlight burns out one evening. What's more, the dark wants to show Laszlo something.