Sarah was four years old when her spirit guide first appeared. One day, she woke up from a nap and saw him there beside her bed. He was short, with longish curly hair, like a cherub made of light. She couldn’t see his feet. They played a board game—she remembers pushing the pieces around—and then he melted away.

After that, he came and went like any child’s imaginary friend. Sarah often sensed his presence when strange things happened—when forces of light and darkness took shape in the air around her or when photographs rippled as though shimmering in the…