"It's been so cool seeing Dad become a grandfather, and to see him interact with our kids," Percy said.
"They're always so excited to see each other -- it's a special relationship we have with our grandparents, and we need to celebrate that while we can."
In Banana Fingers and Fairy Knees, the granddads captured in rhyme are adventurous, hardworking, salt-of-the-earth types full of wit of wisdom.
Their grandkids fondly recall trips in the old ute, building a shed, hunting with a hand-made bow, the "magic words" granddad would mutter while fixing the car and advising them to eat porridge to help them grow.
The poems are set against a rural background, similar to the Pahiatua farm where Percy grew up and where Dale mustered sheep, rode old motorbikes and came up with clever inventions.
The title is a reference to older hands with fingers swollen from their hands-on careers -- but are "very dexterous".
"My Dad has a great set of banana fingers, but plays the piano beautifully.
"So, his fingers dance like fairy feet -- and his knuckles are the fairy's knees."
He said the book was also inspired by his grandfathers, known as Da and Nangnang, especially their fishing trips and putting bait inside "Gran's old pantyhose", and the tales from their youth about stepping in cow pats to keep warm.
Percy said he had no plans to become a children's writer, until the idea for The Old White Truck came to him "one day at 4am" and he read the resulting poem at Mr Mills' wife's 70th birthday.
"Their daughter said everyone could buy copies of the story for $20 -- that forced our hand."
He said he came up with the rhymes for the latest book over a couple of evenings, with Dale then doing the illustrations -- "once he'd got his head around that he'd be drawing some of his own mannerisms".
His next book will likely be based on another unsung hero from Martinborough.
"I love writing about real people, and there so many cool characters around here.They may not have the most glamorous or exciting jobs, but the community can't function without them. It's always great seeing the kids' reactions to the books -- and adults have a bit of a chuckle too."