The author of a book critical of US President Donald Trump’s first year in office insists he had spoken with him, contradicting Mr Trump’s assertion that he had never talked to the writer for the book and had authorised “zero access” to the White House.

“I absolutely spoke to the President,” Michael Wolff said when interviewed on NBC’s Today program on Friday.

“Whether he realised it was an interview or not, I don’t know, but it was certainly not off the record,” Wolff said, adding he had spoken to people who spoke to Mr Trump on a daily, “sometimes minute-by-minute” basis.

Wolff’s book, Fire and Fury: Inside the Trump White House, went on sale on Friday, its release pulled forward after published excerpts this week set off a political firestorm, threats of legal action and an effort to halt publication.

Lawyers for Mr Trump had called on the book’s publisher, Henry Holt & Co, to halt the book’s release, originally scheduled for next Tuesday January 9.

Instead, it began sales early and as of Friday (local time), the controversial book was the top-selling book on Amazon.com, who promoted the author as having “deep access to the West Wing”.

Customers in Washington lined up in bitterly cold weather to purchase a copy at KramerBooks, which began selling copies on Thursday at midnight (local time), reportedly selling out in 20 minutes.

“We had 84 on hand this morning and sold out within minutes,” said Sarah Costello, a spokeswoman for Politics & Prose in Washington. “We have several hundred more on order and were hoping to get them in today.”

Bookstores in other major cities including New York and Los Angeles reported selling out of the initial order, and other shops in places like Oxford, Mississippi, and Shaker Heights, Ohio, also said that buyers quickly snatched up copies of their first batch.

On Thursday evening, Mr Trump tweeted that he “authorised zero access to White House (actually turned him down many times) for author of phoney book! I never spoke to him for book”.

I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don’t exist. Look at this guy’s past and watch what happens to him and Sloppy Steve!

— Donald J. Trump (@realDonaldTrump) January 5, 2018

The book depicts a chaotic White House, a president who was ill-prepared to win the office in 2016, and Trump aides who scorned his abilities.

Asked on NBC to clarify what he meant when he wrote that Mr Trump’s entire circle questioned his fitness for office, Wolff said, “Let me put a marker in the sand here: 100 per cent 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,” he said.

Wolff says he based the book on more than 200 interviews, and had access to the White House and Mr Trump. In defending the contents of the book, he said that he has audio recordings of interviews and extensive notes.

Michael Wolff says President Trump is least credible person "who has ever walked on Earth" in interview with @TODAYshow https://t.co/BM5QQh4Sfh pic.twitter.com/4mvvVaxegW

— NBC News (@NBCNews) January 5, 2018

White House spokeswoman Sarah Sanders said 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 in an interview on Fox News Channel.

But some reporters pointed out inaccuracies. even if they don’t challenge the substance of the book.

Wolff writes that Mark Berman, national reporter for the Washington Post, was among those having breakfast at the Four Seasons in Georgetown one morning early in the administration. Berman said he has never been to the restaurant.

Also listed as being there was Hilary Rosen, “the music industry’s longtime representative in Washington”. She left her post at the Recording Industry Association of America in 2003.

ICYMI: @MichaelWolffNYC had extraordinary access to the White House. Read the first excerpt from FIRE AND FURY in @NYMag https://t.co/BdKxN93HXr

— Henry Holt & Company (@HenryHolt) January 3, 2018

The early excerpts have caused a public rift with Mr Trump’s former chief strategist Steve Bannon over his comments in the book about the President and his family.

Mr Bannon’s reaction to the controversy has been muted.

In interviews after the news broke with the conservative Breitbart News website, he called Mr Trump a “great man” and pledged continued support for the president’s agenda.

So what’s in the book?

Amazon.com’s book blurb describes the first nine months of the Trump presidency as “stormy, outrageous and absolutely mesmerising” with new details about the “chaos” in the Oval Office.

It says the book promises to reveal:

• What the President’s staff really thought of him
• What inspired him to claim he was wiretapped by President Obama
• Why former FBI director James Comey was really fired
• Why former chief strategist Steve Bannon and Mr Trump’s son-in-law Jared Kushner couldn’t be in the same room
• Who is directing the administration’s strategy now
• What the Trump administration has in common with the move The Producers.

In Los Angeles, Book Soup said that it sold out of its initial shipment of the book “first thing” on Friday morning.

Then they sold out of a shipment that arrived on Friday afternoon: “More books Monday”, the bookseller promised on Twitter.

— with AAP