Mr Millen came up with the idea for the book after reading an article on the importance of children knowing names for feelings.
He had previously spent time with children through church programmes, and had an interest in child development.
He wrote much of the book over Easter weekend in 2015, when he was employed full-time as an engineer in New Plymouth.
Crowdfunding provided Mr Millen with the money necessary to self-publish, and he had 200 pre-orders before the book's first print run in early 2016.
He decided to leave work just a few months ago to tour schools and preschools with his book.
"I was at work and just dreaming about the book and thinking, you know what, I want to give it a go," he said.
"For me, as an electrical engineer with no kids, I didn't really have the connections to make it big without leaving my job."
Mr Millen had already spent some time reading to school children while on annual leave.
"I took it to schools - the teachers loved it, the kids loved it and the parents loved it."
In most cases, he said, the schools would buy a copy, along with a number of parents.
"I've got these dreams and aspirations of runaway success but it has been very good so far."
Mr Millen is hoping to publish a new book, the next volume in the series, very soon.
"I'm looking at launching that in the next few months really...I'll be looking at doing another crowdfunding campaign and seeing if there are enough people interested in it."
Jack Feels Big uses te reo and sign language to name feelings as well as English.
He said he is unsure what will happen after he completes the series of books, and has not ruled out a return to engineering.
"I don't know if I'll have more ideas for more books."
The book is available to buy through his website, jackfeelsbig.nz.
