In early 2019, at a memorial service for a prominent psychiatrist, a succession of “doctors and other mental health professionals” told Nancy Pelosi they were “deeply concerned that there was something seriously wrong” with Donald Trump, “and that his mental and psychological health was in decline”.

“I’m not a doctor,” the former speaker writes in an eagerly awaited memoir, “but I did find his behaviors difficult to understand.”

Pelosi’s book, The Art of Power: My Story as America’s First Woman Speaker of the House, will be published next week. The Guardian obtained a copy.

Pelosi was speaker between 2007 and 2011, and between 2019 and 2023, the latter spell coinciding with Trump’s chaotic presidency. Her memoir comes out amid a tumultuous 2024 presidential campaign, in which Trump is the Republican nominee for a third successive election.

Questions about Trump’s fitness for office form a thread through the book. At 78, Trump is the oldest candidate ever, his campaign-trail utterances studied for frequent mistakes, his speeches are often rambling and marked by bizarre references.

Trump’s volcanic behavior and disregard for societal norms also stoke such questions, not least because he left office having been impeached twice, the second time for inciting the deadly January 6 Capitol attack; has been convicted on 34 criminal charges and faces 54 more; has been ordered to pay hundreds of millions of dollars in civil cases including one concerning a rape claim a judge called “substantially true”; and has promised if re-elected to govern as “a dictator” on “day one”.

On the page, Pelosi says she did not solicit statements about Trump’s mental health from attendees at the memorial for Dr David Hamburg, “a distinguished psychiatrist who … served as the president of the Carnegie Corporation, where he had been a great voice for international peace”, and who died in April 2019.

Elsewhere in The Art of Power, however, the former speaker is not shy of stating her views about Trump’s mental health, calling him “imbalanced” and “unhinged”.

By 6 January 2021, Pelosi writes, “I knew Donald Trump’s mental imbalance. I had seen it up close. His denial and then delays when the Covid pandemic struck, his penchant for repeatedly stomping out of meetings, his foul mouth, his pounding on tables, his temper tantrums, his disrespect for our nation’s patriots, and his total separation from reality and actual events. His repeated, ridiculous insistence that he was the greatest of all time.”

She describes how subordinates including Mark Meadows, Trump’s final chief of staff, indulged improper behavior, allowing Trump to “surreptitiously listen” to private meetings with congressional leaders, eventually prompting Pelosi to ban all cellphones from her meeting rooms on Capitol Hill.

Pelosi also describes getting calls from Trump, often late at night, including one in which she says Trump insisted missile strikes on Syria he had just ordered were Barack Obama’s fault, eventually prompting Pelosi to tell him: “It’s midnight. I think you should go to sleep.”

Pelosi devotes attention to the events of 6 January 2021, when she and other congressional leaders were hurried from a mob who meant them harm, then spent hours trying to get Trump to call them off.

Much of Pelosi’s account is familiar, thanks to the work of the House January 6 committee, which she created, and of her own daughter, Alexandra Pelosi, a documentarian who was filming her mother that day.

“People still ask me how I remained so calm,” Pelosi writes, of the hours when Congress was under attack, she and other leaders were evacuated to Fort McNair, and Vice-President Mike Pence was in hiding as the mob chanted about hanging him.

“My answer is that I was already deeply aware of how dangerous Donald Trump was.

“He continues to be dangerous. If his family and staff truly understood his disregard for both the fundamentals of the law and for basic rules, and if they had reckoned with his personal instability over not winning the [2020] election, they should have staged an intervention. Whether because of willful blindness, money, prestige, or greed, they didn’t – and America has paid a steep price.”

Saying she had quickly realised she had “more respect for the office of president of the United States than Trump”, Pelosi says “it was clear to me from the start that he was an imposter – and that on some level, he knew it”.

Still she is not done. After describing how electoral college votes were eventually counted and Joe Biden’s victory confirmed, she says she “and many others wanted a consequence for the deranged, unhinged man who was still president of the United States”.

That led to an impeachment and a second failed Senate trial, after the Republican leader there, Mitch McConnell of Kentucky, made a historic miscalculation: that Trump did not require conviction and barring from office, as he was politically finished.

Pelosi describes another failed effort to remove Trump from office, on grounds of being unfit.

“Following January 6,” she writes, “the Democratic leadership discussed asking the vice-president to invoke the 25th amendment to the constitution, which allows for the vice-president and a majority of cabinet members to certify that a president is unable to discharge the duties of the office.”

She and the Senate majority leader, Chuck Schumer of New York, “placed a call to Vice-President Pence about this possibility”.

Elsewhere, Pelosi writes that she admires Pence for his actions on January 6, when he refused to be spirited from the Capitol despite having to hide from a murderous mob sent by his own president, then ultimately presided over certification of election results.

But when it came to the 25th amendment, Pence let Pelosi down.

“The vice-president’s office kept us on hold for 20 minutes,” Pelosi writes, adding that “thankfully” she was at home at the time, “so I could also empty the dishwasher and put in a load of laundry.

“Ultimately, Vice-President Pence never got on the phone with us or returned our call.”