Let's look at the figures. In 2012, a then 34-year-old American young-adult novelist, John Green, is at the top of the Amazon book charts, before his book, The Fault in Our Stars, is even published. He's also at the top of the New York Times bestseller list.
Roll forward two years and the movie version of this book has just opened in 3,171 screens across the US, and taken $48.2m (£28.4m) at the box office in its opening weekend alone, recouping its budget four times over.
The book is now the bestselling title in the UK, on all formats. It has sold more than nine million copies internationally, and been translated into 47 different languages.
The Fault in Our Stars is about two teenagers who are dying of cancer. They fall in love while they are dying of cancer.
It is a devastating and exquisite read that, startlingly, our children have introduced us to – it's the book that everyone is reading at school, from 10-18 year-olds. Pick it up and you'll be shocked – that a reader, certainly from the younger part of this age range – can handle it, understand it, relate to it.
"John was dealing with a brutal subject matter and wanted to tell a true story," explains Green's editor of 15 years, Julie Strauss-Gabel.
"I was never worried that he was going too far, that I should pull him in. He wanted to show how tough, painful and dehumanising the experience can be – he didn't want to gloss over things."
Not glossing over things is an understatement. The story starts in a cancer support group for children, each person getting up to introduce themselves and their illness, and stage of illness.
The person whose eyes we are seeing this through is 16-year-old Hazel Grace Lancaster (played by Shailene Woodley), our heroine, who has stage four thyroid cancer, and is only alive because of a new drug that should have stopped working on her by now.
At this support group, Hazel lays eyes on a boy with foppish hair and movie-star looks – Augustus Waters (Ansel Elgort) – who is in remission from bone cancer. And thus begins a love story that is dazzling in its innocence and authenticity; the perfect capturing of first love. Screenwriter, Scott Neustadter, says that it is this what he and his writing partner, Michael H Weber, focused on.
"We never really thought of it as a book about illness. It's a love story. The difference between Hazel and Gus, and everyone else, is that they are very much aware that they're going to die and we're not.
"They're very aware of how much time they have left, in a much more tangible way than other people are."
While he was writing The Fault in Our Stars, John Green became a father for the first time. Strauss-Gabel credits this for the reason he writes about unconditional love in the way that he does. The other experience that triggered the book was working as a student chaplain in a hospital, with terminally ill children and teenagers.
Green found the work too difficult, but it began a 12 year-long path to find a way to tell a story about these teenagers who live, love and laugh just like any other teenagers.
"The drive to talk about these young people's issues, what that experience was for them, had been with him from the moment we started working together," explains Strauss-Gable.
The point at which the approach to the story finally solidified was a friendship that Green shared with a young girl called Esther Earl, who had thyroid cancer. The Fault in Our Stars is dedicated to her.
When she died at the age of 16, Green was so angry and frustrated that he poured his feelings into the book.
Suddenly the narrator, Hazel Lancaster, came into being – though not an embodiment of Esther – and there was a story to write.
"John knew, intimately and deeply, what Esther had gone through. She had seen a lot in a short amount of time. Being true to Esther was his priority."
Astonishingly, Neustadter was nursing his own grief when the option to write the screenplay landed on his desk.
"My dad was battling with pancreatic cancer and passed away the week that the book was published.
"There was discussion at Fox over whether to even give it to me. I'm glad they did.
"I was sitting around thinking about cancer anyway. It was cathartic to have something to focus on."
Neustadter and Weber produced a screenplay in six days. They worked from a copy of the book that they each had in their respective homes of New York and LA; highlighting their own favourite bits and sharing out the scenes, which they then emailed to each other.
"It was actually an easy job," admits Weber. "The novel is very cinematic – the characters and their words. The dialogue was already there. John had done most of the heavy lifting." The "heavy lifting" is dialogue that is dizzying in its intellectualising, frankness and wit.
"John is aware of how smart teenagers are, how literate – that they don't want to be spoken down to," Strauss-Gabel explains.
Green has been talking to young brilliant people, via his video blogs with his brother Hank, Vlogbrothers (youtube.com/user/vlogbrothers), for a number of years, and also through his other four novels.
Now with a following of over two million, the VlogBrothers' community call themselves 'Nerdfighters' – youngsters who want to read, debate and heal the world. Green also leads his young troopers in numerous charitable projects that have raised hundreds of thousands of dollars. Esther Earl was one of the first Nerdfighters.
An example of Green's video blogs is 'What to Do with Your Life', from October 2, 2012, or 'How To Become An Adult', from August 31, 2011: "You can't know what an experience will mean to future you until you are future you," Green says to his young fans. "You need millions of seconds of perspective which only time can buy."
They are words that could have come straight out of Augustus Water's mouth; the reason that the characters are breathtakingly engaging in The Fault in Our Stars is that their author is. It was this level of engagement with his young fans that led to pre-orders of 150,000 signed copies of The Fault in Our Stars – Green had to have occupational therapy for his wrist pain.
Strauss-Gabel says youngsters need, and are ready, to address the real world. "Teenagers have always been looking for these stories – to experience the things that worry them in the world.
"In the publishing world we see exceedingly tough subject matters – abduction, slavery, illness. The more we talk about them, and don't hide them away, the better."
That said, it has taken a writer like Green to come along and show Hollywood that there is an appetite for such fearless and brave probing of real life – with no special effects, no super powers and no vampires (ironically, the movie's producer, Wyck Godfrey, is the producer of the spectacularly successful Twilight trilogy).
"But we still didn't really believe that Hollywood would make a movie about a girl with an oxygen cannula, a boy with one leg, and another boy with one eye (Hazel and Augustus' friend, Isaac, who has a rare form of eye cancer)," admits Neustadter.
"And that they wouldn't try and sanitise it.
"But to everyone's credit, the studio said, 'Nope, we're going to go for it, we're not shying away from the ugly underbelly.' They even wanted the cannula on the poster. We couldn't believe it."
The Fault in Our Stars is released on Thursday. The book is available now, Penguin, £7.99
The kids' stories with grown-up appeal
Like The Fault in Our Stars, some of the top movie hits of recent years have come from the sometimes overlooked realms of young adult fiction:
Twilight – the vampire-themed fantasy novels by American author Stephenie Meyer introduced a generation of young readers to protagonists Bella Swan, 104-year-old vampire Edward Cullen (top) and his arch-nemesis, Jacob Black, a werewolf. The books have sold tens of millions of copies and were adapted into five films between 2008 and 2012, starring Kristen Stewart and Robert Pattinson
The Hunger Games – set in a dystopian future America, the trilogy of novels by Suzanne Collins follow the exploits of heroine, Katniss Everdine, as she undergoes a brutal contest in which she must fight to the death against other young people. The film version, starring Jennifer Lawrence (middle), has already proved a smash at the box office, with the final book set for release in two installments this year and next year
The Princess Diaries – written by Meg Cabot, the books were a big hit with young girls for their numerous pop culture references, which include singers, movies, and fads in modern culture. The series was brought to the big screen by Walt Disney Pictures, starring Anne Hathaway and Julie Andrews
Divergent – like The Hunger Games, the novel by Veronica Roth is set in a bleak future (bottom) and follows heroine Tris Prior as she explores her identity within a society that defines its citizens by their social and personality-related affiliation with five different factions. A film version starring Kate Winslet was released earlier this year