When Markus Zusak was writing The Book Thief, he had no idea it would sell more than 10 million copies and spend more than 500 weeks on the New York Times bestseller list.

He will host the “Beyond the Bestseller” session with record-breaking children’s author Andy Griffiths at next month’s Williamstown Literary Festival.

Held on the weekend of June 17 and 18, the Willy Lit Fest, as it is affectionately known, will involve 50 events at the Williamstown town hall and Williamstown library.

Zusak said he thought The Book Thief would be his least successful title because of its dark subject matter.

“Well, it’s set in Nazi Germany, it’s narrated by Death, nearly everyone dies – oh, and it’s 580 pages long, you’ll love it,” he said.

“I honestly thought no one would read it – so yes, its success surprises me to this day.”

He said the book had enabled him to buy a new car – nine years after the book came out in 2005.

“The success definitely didn’t make me feel more confident about my writing – those things never change,” Zusak said.

Griffiths, who wrote Australia’s bestselling book of 2015, The 65-Storey Treehouse, said that when he wrote the first book in the Treehouse series, he knew he was on to something.

The 13-Storey Treehouse was the seventh book in a series of experimental and absurd books that Terry Denton and I had been creating over a decade,” he said.

“We were just like two mad scientists adding a bit of this and a bit of that just to see what would happen.

“We knew we’d created something unique – and with an interesting mix of both gentle and slapstick humour – but we had no idea it would appeal to so many people in so many different countries.”

For the festival program and tickets, visit willylitfest.org.au or call 9932 4074.

Children's book author Andy Griffiths. Photo by Jason South.

Children’s book author Andy Griffiths. Photo by Jason South.