You know they're coming, every time - how many ways can he leave Jack West Jr or Shane Schofield hanging, sometimes literally, only for us to flip the page and have them save the world?

If you're still along for the ride you'll know his last five books in the Jack West series, beginning with Seven Ancient Wonders in 2005, are kind of expansion on that theme; one long-running story that's leading us to, potentially, the end of the world.

With his latest book, The Two Lost Mountains, there's only one corner left to turn as such - he's already writing the final book, one he's dubbed "The One Something Something".

What's going to happen when there is no page to turn on that final chapter?

"I remember when I was younger reading a book, when you got to the end of a chapter and you had to flick the page to find out what happened," Reilly says.

"That to me is the essence of an enjoyable time reading a book, the need to know, and to actually have to flip that page to find out. I make no apologies for doing that.

"But this is a first for me, I have never ended a series before.

"Once I wrote Six Sacred Stones, I knew it was going to be a seven book series, and the 'One something something' will be the end, it will end there.

"My goal with the final book is not only to write something, which is just completely colossal, and gigantic, and just non-stop action from literally start to finish, the whole book is the climax, but I have to make sure it's satisfying.

"And much of that is about Jack, I want readers to be satisfied by the character's end."

I ask upfront if he's going to kill him off.

"Could he die? Yes. How far will I go. You'll just have to wait and see," he says.

He's right. It's probably jumping the gun to be talking about a book Reilly is only halfway through writing, but, as his cliffhanger habit has trained us to do, it seems only fair that we want some instant gratification once we've reached the end of The Two Lost Mountains.

Indeed, The Three Secret Cities, and this latest one, have really been one big story. Cities had a clanger of an ending and he had to throw in a teaser to make us hope he didn't do something totally stupid (he didn't), this one too doesn't offer any real conclusion apart from getting Jack and his team closer to the end of the race to save the world. It's frustrating but you're too busy loving every minute of it to get too angry at him.

But sometimes with a Reilly novel, that is all you need, the action that gets you from A to B.

"Early in my career there was the odd person here or there who would email me and say, 'Dear Mr Reilly, I read your novel and I found it completely unbelievable'.

"One thing I've learned over 22 years is that now those people know not to read a Matthew Reilly book, they know they're not going to enjoy it.

"My readers [and there are millions of them worldwide], are people who get into the spirit of things, who enjoy that suspension of disbelief."

Has that been difficult to maintain?

"It's a constant, battle is the wrong word, but I am very aware that I have to maintain the energy, that the energy I put into the book will come off the page.

"It's constantly in the back of my mind to make sure that I'm still having fun and if I'm having fun, then you're probably going to have fun as well."

Sydney-born Reilly was just 19 when he wrote his first book, Contest. It was rejected by every major publisher in Australia and he self-published it three years later in 1996. Cate Paterson, a commissioning editor at Pan Macmillan found a copy in a bookstore and signed him to a two-book deal.

He moved to Los Angeles in 2015 and still likes it there, despite the politics of the United States. He's close to Hollywood and is working on scripts in between novels. He's coped okay during COVID, the introvert in him has put his head down and has churned out the words.

He's happy that there seems to be a surge in book sales, but he misses going to the movies.

"At times like this I truly believe we need escapism, it's good for our mental health."

He's Skyping from his office, there are a few movie posters on the walls, books and files full of his research on the shelves behind him. That's one thing he's noticed over the years, how he's had to pay more attention to the authenticity of the historical settings and the like, as people are more likely to hit the internet and question his research. He shows me a notebook full of notes and diagrams - "I do love a good diagram" - about how he timelines things and makes sure he's got all the threads from each of the books in the right place. He's had to do it, it's been a winding road.

At 46, he's not the brash, young author anymore, but a man who's measured and thoughtful, while at the same time still a little full of wonder about many things, from popular culture to ancient history.

You sense in the last few books, amidst all the action, there's an underlying theme of the importance of family and what defines that, and how we, as we age, might leave a legacy, and again, what that even means.

"I'm 46, life happens to you, and the one thing I know is that I'm far better now at writing the character elements and the depth of the characters.

"There have been a few big character deaths in the last few books ... and I don't think I could have written those scenes back in my 20s.

"With Ice Station and Area Seven and Scarecrow, it was all about pace, pace, pace, action, action action.

"The Jack West series was always a little different to the Scarecrow books. It was always about this ragtag family of soldiers who are softened by this little girl.

"And so there's a lot more empathy in the Jack West series.

"The best thing I could have done was take that eight year break between Five Greatest Warriors and Four Legendary Kingdoms because I grew up, I grew up a lot.

"I still love writing the big action scenes, but I must say, I enjoy writing the character scenes just as much now."

Fingers crossed, then, that he never writes that scene where we say goodbye to Jack West.