A book about his own mental health crisis, a novel about a 400-year-old long-lifer and the third in a trilogy of children's Christmas books: other than being written by British author Matt Haig, they might not appear to have much in common.

But delve a little deeper and the links become clearer, Haig believes.

Though his earlier books — described as "speculative fiction" — were well-received and Shadow Forest won the 2007 Nestle Children's Book Prize, his deeply personal memoir about mental health, Reasons to Stay Alive, took him to another level.

It's sold more than 100,000 copies since its 2015 publication and, after penning 14 books for adults and children during the past two decades, guaranteed book buyers would sit up and take notice of the Brighton-based author.

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Immediately after finishing it, Haig switched gears and wrote A Boy Called Christmas and last year's The Girl Who Saved Christmas. Centring on a Victorian girl who forms a bond with a youthful Santa Claus, they are intended to appeal to younger readers, although Haig hopes the series can be enjoyed by all.

The third, Father Christmas and Me, is out this month — as Haig watches sales of his latest novel How To Stop Time climb and gets ready for a film adaptation by StudioCanal films with Benedict Cumberbatch to star in the movie.

Cumberbatch will play Tom Hazard, who appears to be 40 but is actually more than 400 years old and has, during the centuries, befriended Shakespeare, Captain Cook and F. Scott Fitzgerald.

Though he's been in similar situations in the past, with talk of his books being adapted for screen, Haig is quietly confident this one will actually make it to the big screen.

"This seems different, as they've got an A-list actor attached to play the main role. That seems to be the way to go in terms of attracting interest, and the fact that the novel's also doing well can only help."

Surprisingly, he sees a link between How To Stop Time, released earlier this year, and his children's Christmas trilogy.

"Father Christmas also knows how to stop time, so time certainly does seem to have become a theme of mine," Haig acknowledges.

"The other similarity is that Father Christmas is also on a journey to find himself and to find his home. A lot of my central characters are people who are on a quest to find themselves, which is what Tom Hazard is doing. Having that miserable background but coming to terms with that and finding the hope in that difficult situation."

Despite its fantastical premise, he also insists How To Stop Time is just as autobiographical as Reasons to Stay Alive.

"It's about someone who has a condition, an invisible condition, where they're carrying around a kind of universal baggage in their heads, while outwardly they look normal," he says, suggesting living with a mental disorder can also affect your perception of time.

"If you're locked in a horrendous state of illness 24/7 with no escape, it makes time drag and stretch. So when I came out of about three years of mental illness that had been near continuous, it felt like I was 400 years old, because it had been so intense. Even now, it seems like that was about half my life, even though it was a relatively small part in real time."

Saying he doesn't want to write endless novels about depression, Haig also needed a change of pace after Reasons to Stay Alive, hence the children's trilogy and How To Stop Time.

"I wanted to write something that would cheer me up, even if the main character, Tom Hazard, is often quite miserable," he laughs.

"He's searching for the meaning of life and he's got a lot of grief. But for me, the experience of writing it was a lot of fun."

Having long lost all of his nearest and dearest and banned from ever falling in love by the rules of the so-called Albatross Society — a secret society made up of other long-lifers — it's little wonder Hazard isn't a particularly happy soul.

Haig acknowledges the story is about dying but also about loss and other people dying because, he says, losing those close to him scares him almost as much as his own death. The father of two believes having children has forced him to consider mortality more carefully.

"Having kids makes you feel older because you're aware of how fast they change, so you literally see time evolving," he says.

"I'm quite a hypochondriac, so I'm always imagining dying of something or other in the next year, so it was quite therapeutic to imagine someone who has lived for centuries and is on this endless quest towards immortality.

"There's also been a lot of scientific developments, such as stem cell research and fiddling about with DNA, which are stretching our lifespans beyond natural levels, which is potentially problematic and quite scary."

From Elizabethan England, where Hazard is taken under William Shakespeare's wing, to 1920s Paris where he crosses paths with F. Scott Fitzgerald, Haig enjoyed cherry-picking his favourite historical periods.

He thought of telling the story as his own personal time machine, which allowed him to go back and write about the times he is most interested in. Rather than travelling a realistic route through history, he wanted to take one that was, for him, more pleasurable. That meant having Shakespeare in the story even though Haig acknowledges any creative writing tutor would advise against it, fearing it was too corny.

Haig was also determined to venture beyond Britain, leading Hazard to join Captain Cook's crew as he explored the Southern Hemisphere in the 18th century. Haig acknowledges that James Cook is seen as a controversial figure in Australia and New Zealand.

"I thought it would be fun to have these real-life people who carry with them these slightly shameful histories."

It's on a voyage Downunder that Hazard first meets Omai, who turns out to share his long-living affliction. Depicted in paintings by Joshua Reynolds and William Parry, the Ra'iatean became the second Pacific Islander to visit Europe when he journeyed to London in 1773.

Introduced to British society by the naturalist Joseph Banks, he was one of the first celebrities to be renowned, in Haig's words, for "just being himself" as he fraternised with well-known figures like Dr Samuel Johnson and Lord Sandwich.

"Omai was a fascinating character and I've slightly fictionalised him," says Haig.

"He became like an exotic trophy from the South Pacific, as the great and the good of London wanted to show off their worldliness by dressing him up in exotic clothes — as you see in those paintings — which weren't anything like his real clothes. Just an exotic idea of what he should wear."

Lowdown
Father Christmas and Me
by Matt Haig
(Allen & Unwin, $23)

How To Stop Time
by Matt Haig
(Allen & Unwin, $33)