The magical system presented in Book of Night is based in part on established psychiatric, spiritual, and folkloric traditions. By drawing on these established cultural beliefs, Black intends her system of magic to feel both authentic and psychologically complex.

In mental health, “shadow work” is a psychological practice that involves facing one’s darkest inner self through methods like journaling, meditation, psychoanalysis, and dream therapy. This technique offers a way of addressing internalized trauma and relies on the work of 20th-century Swiss psychiatrist Carl Jung, who believed that everyone has a “shadow self” where they put their darker, less socially adhering impulses—much like Remy does with Red in Book of Night. The foundation of shadow work is the idea that these inner and outer selves—the persona and the shadow—are in conflict with one another. By using shadow work techniques, one can bring these disparate parts of themselves into balance.

Occult traditions also feature shadows, which form creatures such as the fetch from Irish folklore and the Tibetan tulpa. A fetch is believed to be an apparition of the self—a ghost of the living. Seeing one’s fetch presages death, though fetches could also become magical familiars carrying out the living person’s will.