"There's been a family history, but for members of the extended family, and written by a family member with no footnotes.
"As a professional historian, I need evidential material."
Mr Grant said he was enjoying discovering the life of a "very interesting fellow".
Renall came to New Zealand in 1840, establishing himself in Wellington, initially as a carpenter.
He built Wellington's biggest flour mill at Taita, in the Hutt Valley.
He entered politics, becoming a local body politician and representing Wellington on a national political level.
His mill was badly damaged in an earthquake in 1855, and destroyed four years later in a major flood.
He relocated to Wairarapa, building a mill on the northern edge of Masterton.
At the time, he became involved in an organisation called the Small Farmers Association, based in Wairarapa.
Its intent was to prevent Wairarapa from being taken over by large runholders, ensuring ordinary people could afford to buy parcels of land.
That association eventually became the Masterton Trust Lands Trust.
Mr Grant said Renall was a philanthropist, who gave flour and money to the needy, and bags of flour to the Salvation Army.
His natural bent for politics saw him join the Masterton Borough Council in September 1878, becoming the mayor of Masterton for 1880-81.
"For reasons I don't know, he went off the council in 1881," said Mr Grant.
"I suspect he was easily bored."
He came back to the mayoralty, serving from 1887-89, and staying on the council until 1891, into his late 70s.
He lost the mill to a spectacular fire in 1895 and died "a very old man" in 1902. He was born in 1813.
Mr Grant said Renall was also a Free Thinker, at a time when the practice was quite strong in New Zealand.
"You can't really call them agnostics," he said. "Even though he understood the church, his perspective was that people should have their own thoughts about the hereafter."
He was in good company; New Zealand premiers John Ballance and Robert Stout were also Free Thinkers.
Mr Grant said that, with such a colourful character, careful assessment needed to be made on how much was real.
"He could tell stories, he was a witty bugger. He liked to promote himself, and I think he tended to overstate at times.
"All in all, if you add it up, he's quite a remarkable character."
His send-off, when he died, was huge, he said.
Mr Grant's book will be published by Fraser Books later in the year, with a launch.
"It's been an enjoyable project and I've uncovered a bit more information about Renall that was not known before."
