Mr Furze said there were still another estimated six tonnes of books in the house, but the trust was allowed to sell this lot as a fundraiser.
"Gary had a three-storey home and every room was packed with books, over waist-high. He was bit of with you might call a gypsy book seller. He often put prices on them that were so high it was as if he didn't really want to sell them and wanted to keep them," Mr Furze said.
"In this lot [20,000 for sale] there's something for everybody. The earliest we've found so far [boxes of books were still being unpacked and categorised when The Age visited] are two bibles from the 1860s, and there are a lot of old books, but plenty of new ones too."
He said volunteers helping to sort the books had found more than 500 that were more than 80 years old.
Nicol sourced much of his collection from book fairs, garage sales and second-hand shops over the past decade.
"There are books on everything and a very eclectic mix, from old Enid Blyton books, to modern classics, history, biographies, heaps of books on New Zealand and plenty of fiction," Mr Furze said.
As well as featuring the books on its Facebook page, the trust will also sell them at the Saturday morning markets in Kaitaia.
Money raised would go to the childcare centre and to support the trust's work, which includes advocacy.
Ranginui first opened its doors more than 30 years ago (1985) - making it one of the longest-serving Maori language early childhood centres in the Kaitaia district.
And while its focus is on keeping alive and strengthening Maori language, culture and customs, it is firm believer that any good book is a great thing.
Centre Director Gideon Porter said: "As a kid I was the bookworm of the family, and still am really. So to be able to be a part of this opportunity to spread the joy of many thousands of great books in our community is awesome."
Mr Porter is very appreciative that some of the proceeds will go towards the childcare centre.
To put the 20,000 books in perspective, Kaitaia's public library carries a stock of 25,000 books.