Michael Wolff told BBC radio that his conclusion in "Fire and Fury: Inside the Trump White House"— that Trump is not fit to do the job – was becoming a widespread view.
"I think one of the interesting effects of the book so far is a very clear emperor-has-no-clothes effect," Wolff said in an interview broadcast on Saturday.
"The story that I have told seems to present this presidency in such a way that it says he can’t do his job," Wolff said.
"Suddenly everywhere people are going 'oh my God, it’s true, he has no clothes'. That's the background to the perception and the understanding that will finally end ... this presidency."
Trump has dismissed the book as full of lies. It depicts a chaotic White House, a president who was ill-prepared to win the office in 2016, and Trump aides who scorned his abilities.
Trump took to Twitter late on Friday to renew his attacks on Wolff, and on his former top aide Steve Bannon who was quoted in the book.
"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book," Trump said. "He used Sloppy Steve Bannon, who cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad!"
Bannon, formerly Trump's chief strategist, is chairman of the so-called alt-right Breitbart News website
In his interview with the BBC, Wolff was asked if he believed that Bannon felt Trump was unfit to serve as president and would try to bring him down. "Yes," Wolff replied.
He also hit back at claims that the book was untruthful.
"This is what's called reporting. This is how you do it." he said. "You ask people, you get as close as you can to the event, you interview the people who were privy to the event, you interview other people who were privy to the event, you come to know the circumstance as well as anybody and then you report it."
The explosive book quickly sold out in shops in the US capital, with some lining up at midnight to get their hands on the instant best-seller.
Trump’s mental health
The revelations in Wolff’s book have renewed scrutiny of the Trump’s mental health.
Although the White House has denounced the book as "complete fantasy”, the book sheds light on concerns among top White House aides over Trump’s psychological fitness for America’s highest office.
"Everybody was painfully aware of the increasing pace of his [Trump’s] repetitions,” Wolff wrote.
"It used to be inside of 30 minutes he’d repeat, word-for-word and expression-for-expression, the same three stories – now it was within 10 minutes. Indeed, many of his tweets were the product of his repetitions – he just couldn’t stop saying something.”
The sense of urgency surrounding Trump’s mental state even led Bandy Lee, an assistant clinical professor at the Yale School of Medicine, to brief a dozen members of Congress last month on the potential risks associated with the president’s behavior.