Trump's lawyers had tried to block publication of 'Fire and Fury: Inside the Trump White House', alleging it contained many falsehoods.
The president himself tweeted today that the "phony new book" was being pushed by the media and others to hurt him. He added: "They should try winning an election. Sad!"
But Wolff says he appreciates all the free promotion from the president.
"Where do I send the box of chocolates?" Wolff asked playfully in his first interview about the book, which became the country's hottest book in the last two days.
"Absolutely," Wolff said, and "he's helping me prove the point of the book."
Wolff told "Today" on Friday that he "absolutely spoke to the president" while working on the book.
"Whether he realised it was an interview or not, I don't know, but it certainly was not off the record," Wolff said. "I've spent about three hours with the president over the course of the campaign, and in the White House. So, my window into Donald Trump is pretty significant."
But Trump says he has not spoken to Wolff.
"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," Trump tweeted.
The book cites former top White House aide Steve Bannon as describing a meeting at Trump Tower in New York between a Russian lawyer and Trump election campaign officials, including Trump's son Donald Jr, as "treasonous".
It also portrays Trump as being surprised at winning the presidency last year.
Wolff's book makes many claims, including that his daughter, Ivanka, had a plan with her husband, Jared Kushner, that she would be "the first woman president"
White House aides have depicted the book as a work of fiction.
Wolff says he has receipts: "I work like every journalist works. I have recordings, I have notes. I am certainly and absolutely, in every way, comfortable with everything I've reported in this book."
Wolff has a controversial track record. He has racked up scoops over the years, but critics have at times accused him of sloppy or unethical reporting practices.
The book soared to No. 1 on Amazon's best selling books list on Wednesday and has remained there ever since, CNN reported.