Donald Trump wrote a “shockingly gracious” letter to Joe Biden on leaving office, a new book says, amid the unprecedented disgrace of a second impeachment for inciting the deadly Capitol attack as part of his attempt to overturn Biden’s election victory and hold on to power.

According to excerpts published by Politico on Tuesday, The Fight of His Life: Inside Joe Biden’s White House, by Chris Whipple, captures Biden saying of Trump’s note: “That was very gracious and generous … Shockingly gracious.”

Presidents traditionally leave letters for their successors. George HW Bush’s note for Bill Clinton is generally held up as an ideal of civility between presidents from different parties.

After Bush died, Clinton wrote in the Washington Post that the letter revealed “the heart of who he was … an honorable, gracious and decent man who believed in the United States, our constitution, our institutions and our shared future”.

Trump refuses to admit Biden beat him fairly, faces extensive legal jeopardy for his election subversion attempts, and recently called for the constitution to be “terminated” so he could return to power.

Biden has said Trump’s letter was “very generous” but he has not shared its contents. According to Bob Woodward and Robert Costa, authors of the book Peril, on discovering the note in the Resolute Desk in the Oval Office, Biden “put it in his pocket and did not share it with his advisers”.

Whipple’s book will be published in January. He told Politico writing it was “tough, because … this is the most battened-down, disciplined, leak-proof White House in modern times”.

But Whipple’s previous books include The Gatekeepers, about White House chiefs of staff, and access to the Biden White House included interviews with Ron Klain, the current holder of that post.

Whipple told Politico: “I think Biden’s presidency is the most consequential of my lifetime. His legislative record is comparable to [Lyndon B Johnson’s] and he’s been underestimated every step of the way. But it’s also been a tale of two presidencies – the first year and the second year.

“What makes this such a great story is that Joe Biden and his team really turned it all around, I think.”

Regarding comments released as reports said Volodymyr Zelenskiy, the president of Ukraine, was on his way to Washington to speak, Politico said Whipple cited Biden’s response to Russia’s invasion of Ukraine as well as domestic successes as proof for his contention that the president had turned things around.

Whipple interviewed White House staff on “deep background”, allowing quote approval, and conducted written interviews with Biden and Kamala Harris, the vice-president. According to Politico, Harris left some questions blank, while Whipple’s book reports her dissatisfaction with her role and dissent within her team. Biden, Whipple says, initially considered Harris “a work in progress” as vice-president, the office he held for eight years under Barack Obama.

Whipple also writes that Biden “felt let down by his briefers” over the US exit from Afghanistan, which was widely held to be a disaster when it took place in late summer 2021. Politico quoted William Burns, the CIA director, Mark Milley, the chair of the joint chiefs of staff, and the secretary of state, Tony Blinken, debating the role of US intelligence assessments.

A White House spokesperson said: “We respect that there will be no shortage of books written about the administration containing a wide variety of claims. We don’t plan to engage in confirmations or denials when it comes to the specifics of those claims. The author did not give us a chance to verify the materials that are attributed here.”

Politico also reported a direct comment from Klain – to Whipple via text message. Many observers including reporters for Politico expected Biden to suffer a shellacking in the midterm elections last month. In the event Biden and his Democratic party did unexpectedly well, losing the House but only narrowly, holding the Senate and winning key state races.

At 1.16am on Wednesday 9 November, the day after election day, Klain texted Whipple to say: “Maybe we don’t suck as much as people thought … Like maybe the nattering negatives who dumped to Politico were wrong!”