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.

I am far from the only one to be inspired by imaginary friends, so here is a list of stories from the borderland where the self ends and something else – somehow – begins.

1. Alice’s Adventures in Wonderland by Lewis Carroll

This classic children’s novel from 1865, takes a tumble down the rabbit hole to a surreal land of make believe. Here Alice encounters a whole cast of imaginary beings – some more friendly than others – from the white rabbit that sets it all in motion, to a mad hatter and a vanishing cat. She ends up in the court of the despotic Queen of Hearts, who commands a whole deck of animated playing cards. Even though Alice’s adventures are revealed to be a dream, the story leaves behind a lingering sense of doubt.

2. The Pisces by Melissa Broder

Lucy, a PhD candidate with mental health issues, goes to Los Angeles to house-sit for her sister. While there, she meets and falls in love with a merman, and as Lucy believes that only a man can save her from herself, he seems to be the answer to her troubles. This might have led to happily ever after in another kind of novel, but their relationship meets some challenges, with the merman being a fish and all. This novel is gritty, sad and surreal, but also very funny.

3. Pale Fire by Vladimir Nabokov

Posing as a posthumously published poem by the fictional poet John Shade, with commentary by his neighbour Dr Charles Kinbote, this masterpiece of novel is not what is seems. Through the increasingly disturbing and fantastical commentary, it soon becomes evident that Kinbote is not well. His identity is a slippery thing, and he nurtured an unhealthy obsession with Shade. This novel is a puzzle of a story, brilliantly written and utterly baffling.

4. Among Others by Jo Walton

Fifteen-year-old Mori is sent to boarding school after her twin sister dies in a crash involving their mentally ill mother. Two things helps Mori get through the days: her books and the bond she has – or believes she has – with nature’s fairies. Among Others is a fantasy novel, but the magic is so subtle and intertwined with reality that it is hard to say where one thing ends and the other begins – or if the magic is there at all.

Intriguing … Edward Norton in David Fincher’s 1999 film of Fight Club.

Photograph: Allstar/20th Century Fox

5. Fight Club by Chuck Palahniuk

Most people are familiar with Joe, his enigmatic friend Tyler Durden and their secret basement clubs, if not from Chuck Palahniuk’s book then from the 90s movie starring Brad Pitt. I may not find the novel as philosophically interesting as many others do, but I think it is a highly entertaining take on an unravelling mind. Joe is a very unreliable narrator, and I questioned absolutely everything by the end, which is just the sort of thing I like.

6. The Good People by Hannah Kent

Not so much a novel about imaginary friends as imaginary foes. Based on a true story, it tells the story of Nóra who lives with her disabled grandson in a rural village in 1820s Ireland. After Nóra’s husband dies suddenly, people start whispering about fairies, and suggest that the boy is a changeling, responsible for everything that goes wrong in their community. The novel explores the dangers of ignorance and the power of belief, and how those things combined can make good people do unthinkable things.

7. The Other by Thomas Tryon

This classic horror novel about a pair of twin boys is set in 1930s Connecticut and is hard to discuss without giving away the twist. Niles and Holland Perry may look the same, but they are very different boys. Holland is outgoing and adventurous, sometimes even cruel, while Niles is the thoughtful and compassionate one. When horrible events start taking place, it is obvious who is responsible – or is it? The lush, atmospheric writing is a stark contrast to the chilling events it records.

8. The Boy Who Could See Demons by Carolyn Jess-Cooke

Child psychiatrist Anya Molokova meets 10-year-old Alex after his mother’s fifth suicide attempt. Alex is a normal boy, but his best friend, Ruen, is a 9,000-year-old, shape-shifting demon. Anya naturally assumes that Alex is mentally unwell, but during the course of his treatment, Alex tells her things he could not possibly know. The novel courts the supernatural without ever losing its grip on reality, which is part of what makes it so enticing.

9. The Tall Man by Phoebe Locke

Possibly inspired by the slender man myth, this novel is about mother and daughter, Sadie and Amber Banner, and how their lives are affected by Sadie’s teen entanglement with a group of girls obsessed with a local legend. Told from different timelines and from different points of view, the novel describes the horrible events that lead up to Amber becoming a murder suspect. The Tall Man is a fast-paced mystery with a touch of whodunnit that may, or may not, have a supernatural monster at its core.

10. The Haunting of Hill House by Shirley Jackson

This is my all-time favourite, and I felt strange for a week after reading it for the first time. In this novel, the imaginary friend is the house itself, which takes the emotionally frail protagonist, Eleanor, under its twisted wings, and seduces her with promises of home and acceptance. As with many of the other great books in this genre, it becomes increasingly unclear where Eleanor ends and the house begins, or even if they were divided to begin with. Heartbreaking, immersive and not sane, The Haunting of Hill House is one of a kind.

• You Let Me In by Camilla Bruce is published by Transworld (£12.99). To order a copy go to guardianbookshop.com. Free UK p&p on all online orders over £15.

