https://www.theguardian.com/books/2020/feb/26/top-10-books-about-imaginary-friends

From Lewis Carroll to Vladimir Nabokov and Shirley Jackson, the best of these stories combine fantasy with very real psychology

In fiction, the imaginary friend lives where fantasy, mental illness and the supernatural meet, and it is often intriguingly hard to tell just where it belongs. Children’s imaginary friends are often endearing, as seeen in the countless stories about favourite toys that come to life. But once the protagonist is adult, the imaginary friend can become a sinister presence – a warning that something is wrong. Sometimes it is the relationship itself that is imagined, in fiction as in reality, as when a stalker is convinced they share a special bond with their prey. There is also the eerie notion that a reader’s sense of closeness to a fictional character is a form of imaginary friendship as well.

My debut novel, You Let Me In, is about Cassandra, whose whole life has been influenced by her invisible friend, Pepper-Man. He can provide comfort and protection, but he can also be dangerous, to Cassandra herself and to people around her. It is not so easy, however, to figure out exactly who, or what, Pepper-Man is. He could be a fairy from the forest, as Cassandra firmly believes, or a creature that lives solely in Cassandra’s mind, as her psychiatrist is just as convinced.