"Fear" is virtually assured of joining Michael Wolff's "Fire and Fury" as a million-selling takedown of Trump.
Barnes & Noble, meanwhile, said "Fear" has had the "fastest sales for an adult title" since July 2015.
"The contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence", Mattis explained. In 2004, Invoice Clinton's memoir "My Existence" sold larger than 1 million copies within eight days.
Former White House staff secretary Rob Porter and onetime economic adviser Gary Cohn both pushed back against "Fear", which portrays a White House mired in dysfunction, with aides disparaging the Republican president and working to prevent him from making disastrous decisions.
Woodward, who helped uncover the Watergate scandal and serves as an associate editor at The Washington Post, backed up his anonymous sourcing by saying he conducted hundreds of hours of interviews with witnesses inside the White House, as well as meeting notes, personal diaries and government documents.More news: Super Typhoon Forecast to Hit Philippines Saturday
More news: Storm Florence prompts Trump to declare a disaster
More news: As Apple launches new iPhones, here are flagships it may have discontinued
Throughout the book, Woodward describes a "nervous breakdown" within the Trump administration, detailing high-level administration executives' attempts to tame the impulsive, mercurial leader President Trump appears to be.
Bob Woodward's "Fear" is spreading quickly.
Eric Trump spoke out about Woodward's anti-Trump book and said, "Don't you think people look through the fact that you can write a sensational, nonsense book, CNN will definitely have you on there because they love to trash the president".
The president's son dismissed the allegations in Woordward's book and said he wrote it to make "three extra shekels", and was subsequently accused of being anti-Semitic. Woodward is not Jewish.
Woodward's yarn of a dysfunctional Trump White Home has already sold larger than 750,000 copies, Simon & Schuster launched Wednesday, a day after the guide arrived in stores.