"A person who treats women like they're pieces of meat, who lies constantly. and insists the American people believe it, that person's not fit to be President of the U.S., on moral grounds", he added.
In the book, Comey said the president asked him to be loyal, writing: "This President is unethical, and untethered to truth and institutional values".
The sense that the clash between a disruptive President who marches to his own tune and traditional establishment values is becoming serious heightened over the weekend after Comey, who was sacked by Trump, purportedly for not professing loyalty to the President, attacked him for moral turpitude and suggested he may be compromised because of this.
Trump's tweet came the morning after Comey gave his first televised interview since he was unceremoniously fired last May while overseeing the FBI's investigation into Russian interference in the USA election.
In the Monday tweet, Trump accused Comey and former Deputy FBI Director Andrew McCabe, whom he also fired, of being "Disgruntled", alleging they and other FBI officials collectively "committed many crimes!"
Comey discussed the decision-making that went into announcing just days before the election that the FBI was re-opening the Clinton email investigation.
Former FBI special agent and 25-year veteran Jack Garcia was particularly scathing after Comey's interview. "This president is not able to do that", he said. "These are more words I never thought I'd utter about a president of the United States, but it's possible".
Meanwhile, as for what the President might have been seeking to cover up, Mr Comey says "it's possible" Mr Trump has been compromised by the Russians, although he still thinks it's unlikely. I don't think he's medically unfit to be president.
Patrick Cotter, a former federal prosecutor, said that while there is less risk in limiting a witness' public statement about an ongoing case, Comey's story already has been the subject of a full, public Senate Intelligence hearing.
Comey didn't limit his criticisms to Republicans, saying that some of President Obama's behavior surrounding the Clinton email scandal baffled him as well. "I think he felt a pressure in the political environment because he wanted Hillary Clinton to be elected, to give her a shot in the arm".
In the interview that aired Sunday, Comey said that he would not favor impeaching Trump to remove him from office, because that "would let the American people off the hook and have something happen indirectly that I believe they're duty-bound to do directly" - meaning through elections.
"He looked slightly orange up close", said Comey.
"I hope one of the things that comes out of the Trump administration his first term is a recognition that, as much as we fight about those policy issues in this country, what's at the core of this nation", Comey said in the interview.
Comey's firing came as the Federal Bureau of Investigation was probing possible connections between Trump's 2016 presidential campaign and Russia's meddling in the United States election. In it, Comey wrote that Trump, in a private meeting, pressed the then-FBI director for his loyalty.
'I was trying to be honest and clear with the American people. "I hardly even knew this guy".
The Republican Party set up a website to attack the book, branding him "Lyin' Comey".