"I've had girls tell me about their issues with eating disorders, suicide, and relationships and writing this book gave me the chance to be there for them and to offer them hope with how much I suffered as a teenager," says Delevingne.
Although critics have questioned the parallels with Delevingne's own life (her protagonist deals with alcoholic parents, Delevingne's mother struggled with drug addiction), she's quick to shut down that line of questioning.
"The book isn't autobiographical in any sense. Yes, I grew up in London and yes, I was a drummer and yes, I had a rough time as a teenager, so while it is inspired by my life, none of it is based on fact."
That inspiration was pulled from Delevingne's teenage diaries, which would probably have yielded enough material for several books.
"I always felt alienated growing up and I didn't see my life going anywhere. I didn't do as well as other people at school, which made me hate myself."
I'm not allowed to ask about her personal life (onr her family or famous friends) but Delevingne's mental health struggles are well-documented: there was an eating disorder at 11, depression and a breakdown at 15, suicidal thoughts and self-harm (she would run into trees to knock herself out, or scratch herself until she bled).
Delevingne has managed to lasso these demons into the book, which she co-wrote with best-selling British author Rowan Coleman.
But how much did she actually write, and how much was Coleman's work?
"I'm not sure how many words we each contributed but the ideas were all mine. I had the final say on the story but Rowan helped to bring it to life."
For much of the book's gestation, Delevingne was in Toronto filming Life in a Year, in which she stars with Will Smith's son, Jayden, so most of the copy was put together via email.
"We had a pretty intense email relationship for over a year. But Rowan was fantastic to work with and she has a 16-year-old daughter, so she knows all about the teenage angst thing."
It's ironic that Delevingne, who was probably voted "least likely to become an author" at the posh British school she attended, has now done just that. She suffers from dyspraxia, a developmental co-ordination disorder, which meant school was particularly challenging.
"My brain moves a lot faster than my ability to write, so in exams I would never do very well. I was made to feel bad about that."
There is, however, much in Delevingne's life not to feel bad about. Her father is a property developer, her mother was a model whose parents owned a publishing empire (her grandmother was a lady-in-waiting to Princess Margaret), her godmother is Joan Collins and home was a mansion in Belgravia where she was too busy watching television to speak to Madonna when she popped over.
With her genetically gifted looks and those lush, caterpillar-like eyebrows, it was clear modelling was going to find her. Delevingne has since clothes-horsed for an A-Z of couture houses including Burberry, Chanel, Fendi and Victoria's Secret. In 2014 modelling tipped over into acting, with roles in the films Paper Towns, Suicide Squad and last year's Valerian and the City of a Thousand Planets.
However, Delevingne says she found the process of writing this book so cathartic she'd love to do another.
"The message of Mirror Mirror is that even in your darkest moments, there's always hope if you believe in yourself. I know that more than anyone and I have a lot more to say about it."
Mirror Mirror
BY Cara Delevingne (Hachette, $20) is available now.
