In an interview on NBC's "Today" program Michael Wolff stood by his reporting and said he had talked to President for the book, which Trump has dismissed as full of lies.
The author of a tell all book on Donald Trump's first year in office has spoken out for the first time since explosive excerpts were leaked this week.
Michael Wolff contradicting the US President's assertion that he had never talked to the writer for the book, and had authorised "Zero access" to the White House.
"Fire and Fury: Inside the Trump White House" went on sale on Friday, its release pulled forward despite threats by Trump lawyers of legal action and an effort to halt publication.
Wolff told NBC that he stood by his reporting and had talked to the president for the book.
"I absolutely spoke to the president. Whether he realised it was an interview or not, I don't know, but it was certainly not off the record," he said on NBC's "Today" program, adding that he had spoken to people who spoke to Trump on a daily, "sometimes minute by minute" basis.
Asked to clarify what he meant when he wrote that Trump's entire circle questioned his fitness for office, Wolff said:
"Let me put a marker in the sand here: 100 percent of the people around him ... They all say he is like a child."
"And what they mean by that is he has a need for immediate gratification. It's all about him."
Early excerpts of the book also caused a deep public rift with Trump's former chief strategist and top campaign aide, Steve Bannon, over his comments in the book about Trump and his family, shaking the president's populist movement.
Lawyers for Trump had called on the book's publisher, Henry Holt & Co, to halt the book's release, originally scheduled for Tuesday.
Instead, it began sales early.
As of Friday, "Fire and Fury" was the top-selling book on Amazon.com.
Customers in Washington queued up in sub-freezing temperatures to purchase a copy at KramerBooks, which began selling copies at midnight. The bookstore had sold out of copies as of Friday morning at 9 am.
The White House has said the book is riddled with errors.
Spokeswoman Sarah Sanders, who has called the book trash, said again on Friday that Wolff had talked only briefly to the president and did not have wide access.
"This is a guy who made up a lot of stories to try to sell books," she said.