Melbourne-based, New Zealand-born IT consultant-turned-author Graeme Simsion’s mega-successful Rosie books have sold almost five million copies in 40-plus countries since The Rosie Project in 2013.

Readers fell in love with genetics professor Don Tillman who developed a 16-page survey to find the perfect wife.

With tech industry giant Bill Gates among the books’ biggest fans, was the pressure to land the third and final instalment, The Rosie Result, overwhelming?

En route to sign piles of the new book, 62-year-old Simsion tells The New Daily that it was – but for a very different reason.

“I felt quite a lot of pressure about getting the autism stuff right, because in the first two books, we skirted around the issue, but in this one, we’re confronting it head on,” he said.

Centred on the challenges facing now-married Don and Rosie’s son, Hudson, who demonstrates many of his father’s socially awkward traits and isn’t fitting in at school, autism is raised as a very real possibility.

Simsion felt under huge pressure to get the details right, particularly as he’s not autistic himself.

“I’m putting something out there as entertainment, but if I pitch this wrongly, I could really upset people and do harm, so you do feel a sense of responsibility there.”

He’s heartened by how much the conversation has changed over recent years, aided by the likes of TV blockbuster The Big Bang Theory and the novel, The Curious Incident of the Dog in the Night-Time.

Graeme Simsion with wife and fellow author Anne Buist. Photo: Supplied/Text Publishing

“That’s where people get their impressions [on autism] from, more so than any learned paper.

“When the first book came out, I remember a respected journalist asking me, ‘so Don suffers from Asperger’s disease?’” Simsion says. “Now we don’t really talk about Asperger’s anymore, and we certainly don’t talk about it as a disease.”

Thankfully the books have been embraced by the autistic community, to whom Simsion dedicates this latest, loving look at a family contending with their son’s unique outlook and how others react to that difference.

A father of two fully-grown adults, he put a lot of his early dilemmas into the book.

“Being a father is being a father, whether your child is autistic or not,” Simsion says. “You’ve got to face this question: how much do we seek to mould them, and how much should we let them be themselves?”

He also drew on his own memories of being an 11-year-old in his native New Zealand to get the basics right with Hudson.

“There’s probably more of me in Hudson than there is in Don,” he notes.

Simsion’s wife and fellow author Anne Buist, with whom he co-wrote last year’s Ellen DeGeneres-optioned Two Steps Forward, and who always helps him plot his latest work, inspired this trilogy, bridged by 2016’s The Rosie Effect, in her own way.

“My publisher Michael Heyward said it’s unusual that [the Rosie books] are about a happy marriage, and while I didn’t base Rosie on Anne, I certainly drew on our happiness together.”

Both successful professionals before they turned to writing, Simsion says life together hasn’t changed all that much since the books’ popularity exploded.

“We don’t live our lives flying on Lear jets, and we don’t aspire to that.”

While he used to travel the world talking about data modelling, now he gets to do the same, but talking about his writing.

“That’s what’s really important, that I get to write for a living, and that’s the massive change.”

The Rosie Result, by Graeme Simsion, Text Publishing, RRP $30