Halfdan W. Freihow has been a Norwegian literary critic and publisher for years, but it took him quite some time to decide how best to tell the story of Gabriel, his autistic son.

"Five years was the time I spent trying to figure out how to write about him," Freihow, in Toronto for the International Festival of Authors, said in an interview about his poignant book "Dear Gabriel: Letter From a Father."

"I wasn't really, in the beginning, thinking about publishing it as a book, it was more a question of me wanting to write . . . in order to understand him," the 48-year-old writer recalled.

"I struggled with many writing strategies - novel forms, non-fiction forms - but then, when I finally landed on the form of a letter, it went quite quickly because this had been going on in my head for several years."

Freihow's book, with its fitting setting of an isolated Norwegian seaside home, is many things: a deeply personal look at the complicated aspects of autism and his own struggles to understand it, and a loving tribute to his boy's dogged need to maintain order in an impossibly unorganized world.

"Dear Gabriel," published three years ago in Norway and now in Canada by House of Anansi Press in an English translation, is also timely: autism rates around the world are rising, although it's unclear whether the disorder is more prevalent today or is getting diagnosed more than in the past.

An estimated 190,000 Canadian children have the disorder, with the most recent epidemiological studies suggesting the rate in Canada has climbed to 60 cases per 100,000. Some have suggested that environmental factors could be playing a role in the hike.

It's a disorder that Freihow has come to know intimately since Gabriel's birth 12 years ago. The boy, the youngest of four children, was diagnosed at age three.

"The world for him is planned - this and then this is supposed to happen, and if one element in that plan falls away, it's as though his whole world crumbles and he has to restructure it," Freihow says.

"He doesn't feel the kind of disappointment that a normal child would feel, but more a loss of order in the world and he has to sit down and recreate a trustworthy order. Sometimes I am so impressed at the amount of mental energy he has to use every day just to keep a coherent world image around him."

Gabriel, at nine years old, learned to read while Freihow was writing the book, but the author says "Dear Gabriel" wasn't something his son could easily grasp because it delves into so many abstract concepts about reasoning and perception.

"There was just sort of a key he had to find to unlock the secret of reading and he did, and all of a sudden he started reading everything," Freihow remembers.

"But even though it's written as a letter to him, it is written in a language that is not very easily accessible to a child. It was made into an audio book which I read and he listened to half of it, and then he just let it go ... In a way, he's distanced himself a bit from it. That was a process that started when he realized this was actually not only a letter to him, because then it would have been enough to print one copy."

His devotion to Gabriel has been such, Freihow points out, that it was difficult finding the time to both write the book and promote it.

"I had always kept a notebook with me, and everything he says in the book is a direct quote," he says. "I spent a few winter months in Italy and wrote the book. ... It was very hard at home to sit and do a long stretch of writing, because he will demand attention."

He noted the irony.

"To write about him, I have to be away from him. And then after publication, to talk about him, I have to be away from him again. He didn't really understand why he couldn't come to Canada with me because, after all, this was his book, his letter."