WASHINGTON — Around Labor Day in 1998, the historian Michael Beschloss took his friend, Jon Meacham, then a Newsweek writer, to Kennebunkport, Me., where Mr. Beschloss interviewed George Bush, the former president, at the family compound. After their conversation, Mr. Bush took both visitors on one of the hair-rising rides he loved to take on his speedboat.
That was the beginning of an unusually close relationship between Mr. Bush and Mr. Meacham, who went on to be a Pulitzer Prize-winning author, historian and, eventually, Mr. Bush’s biographer. In 2006, the former president granted Mr. Meacham access to his extensive diaries, both audio and written, and as their connection grew, the project evolved into “Destiny and Power: The American Odyssey of George Herbert Walker Bush,” a No. 1 New York Times best seller in 2015.
Another measure of that deeply personal connection — unusual for most historians and their subjects — will be on display Wednesday when Mr. Meacham delivers a eulogy for Mr. Bush at his funeral at Washington National Cathedral, one of four men the former president chose to perform that duty.
Initially, members of the Bush family were skeptical of Mr. Meacham, in part because of lingering ill will toward Newsweek magazine, which in 1987, before Mr. Meacham worked there, ran a cover story on Mr. Bush with the headline “Fighting the Wimp Factor.” It did not help that he was a frequent commentator on MSNBC. But he eventually won them over.
“I think Jon has all along made clear that he is someone who views the president with admiration,” said Nancy Gibbs, co-author of “The Presidents Club: Inside the World’s Most Exclusive Fraternity” and former editor of Time. “That didn’t mean he was uncritical. The Bushes agreed to cooperate and give him the access they did. They knew and trusted that he would be fair, not that he would be flattering. And both lived up to it.”
“They trusted him enough to be revealing,” she added, “and he respected them enough to honor their trust.”
By first listening to hundreds of hours of Mr. Bush’s diaries, from thoughts in the Oval Office to late-night ruminations, Mr. Meacham was able to get a rare, real-time feel for his subject’s thought processes and decisions. He then spent hours interviewing Mr. Bush and those closest to him.
While the book is largely positive, Mr. Meacham was also critical of Mr. Bush’s tactics in the 1988 presidential race, his role in the Iran-contra affair and some of his domestic policies. Barbara Bush, the former president’s wife, was known to keep a list of the parts of the book she did not like. Yet Mr. Meacham also delivered a eulogy at her funeral.
“It’s pretty rare that you have a biography done by someone who is sympathetic but at arm’s length and had about as full access to a source as an author could get,” Mr. Beschloss said.
Mr. Meacham, who declined to discuss his relationship with the former president, has said that the essential challenge of the book was to explain the gap between the president’s public and private personas, and he had the research to be able to do that.
“When you are writing about a president, what you want is sources that show you what he was like on the inside and how that was different from the public view,” Mr. Beschloss said.
After his book was published three years ago, Mr. Meacham made another visit to the former president in Kennebunkport. With his health failing, Mr. Bush was known to ask friends to read chapters of the book to him, including Mr. Meacham. Listening to the passages, the famously modest former president’s reaction was, “That’s an awful lot about me.”
That modesty is likely to be one of Mr. Meacham’s themes on Wednesday. In an Op-Ed article in The New York Times this month, he suggested some of the others.
“Mr. Bush was a gentleman, but he was a politician, too, and therein lay the great tension of his life,” Mr. Meacham wrote.
He noted that the arc of Mr. Bush’s political career tracked many of the changes that were transforming the Republican Party, and that while he “struggled to govern like Ike, the world around him was beginning to resemble a Joe McCarthy rally.”
Mr. Meacham’s most recent book, “The Soul of America: The Battle for Our Better Angels,” published in May, has been described as a historical rejoinder to the coarseness and harsh edges now characteristic of the Trump era. And it will not be surprising if he sounds those themes in his remarks.