By Agence France-Presse
The highly-anticipated book, released April 17, debuted at the top slot on Amazon's bestseller list, but by Tuesday had slipped to second place.
More than a million copies of "A Higher Loyalty: Truth, Lies and Leadership" have been printed to date, Flatiron Books said through a spokesperson.
The book is available in print and digital formats.
Despite the blockbuster figures, fueled in part by his barrage of media interviews and a high-profile book tour, Comey's work is not even the bestselling political tell-all of 2018.
"Fire and Fury," the explosive tell-all by journalist Michael Wolff, which also focuses on Trump, had a stronger start in January, with over one million copies sold in its first week, according to his publisher.
So far, Wolff's controversial account of life in a chaotic White House has sold over two million copies, according to The New York Times.
The sales figures are welcome news for Macmillan Publishers, the parent company of Flatiron Books and Henry Holt and Company, which published "Fire and Fury."
In the book, Comey, 57, writes at length about his life as a federal prosecutor and senior Justice Department official, the cases he encountered during his career, and his thoughts on "ethical leadership."
But he also addresses his relationship with Trump, whom he portrays as dishonest and egocentric. He compares the president's methods -- particularly his demand for loyalty -- to those of a mafia boss.
The book, which was also in the top 20 in Britain, Germany and Australia on Tuesday, has infuriated Trump, who blasted its author as an "untruthful slimeball" on Twitter.