Joe Biden, the former vice president, was four minutes and forty seconds into discussing his new book, Promise Me, Dad, when he got snagged on a memory. We were sitting in the den of his vacation home, in Rehoboth Beach, Delaware. It was a hot day in late summer, and while his wife, Jill, and his sister, Valerie, milled around nearby in casual workout gear, Biden was smartly attired in a checked dress shirt, charcoal trousers, and black tassel loafers worn without socks—as if primed for an afternoon of shirtsleeve campaigning. In his genially raconteur-ish Uncle Joe way, he recalled how eye-opening it was, 25 years ago, to read Richard Ben Cramer’s What It Takes, a chronicle of the 1988 presidential election that is considered a modern classic of political nonfiction. Biden was one of six candidates whose campaigns granted Cramer nearly unfettered access. When the book came out, in 1992, Biden told me, he spent four hours discussing it with another of the ‘88 race’s also-rans, Senator Bob Dole: “I said to him, ‘You know, I looked at it, and there’s things in there I don’t like, but I can’t say they’re not true.’ ”

Writing Promise Me, Dad, to be published by Flatiron Books on November 14, forced another of these reckonings—only, this time, Biden found, he had to be his own Cramer, confronting truths about his life that he had heretofore blocked out. “I realized,” he said, “how I engaged in the willing suspension of disbelief. How, until I had to write it down, I could not let myself think about the really bad parts about Beau—illness.”

Beau. Illness. This is where the snag happened. Biden’s eyes suddenly flashed and reddened, as if he was seeing something in his mind that he didn’t like seeing, and he bowed his head for a moment. The reason I know the precise timing of this is that I instinctively did the same and, in so doing, caught sight of my recorder on the coffee table, its L.C.D. readout blinking 4:40. Joseph Robinette Biden III, the firstborn of Joe Biden’s four children, known as Beau, died of brain cancer on May 30, 2015, at the age of 46. More than two years later, and less than five minutes into an interview, his father’s grief was still quick to surface.

But not for long. Biden paused briefly, swallowed, looked up, and calmly resumed talking. Finishing his thought, he described how his second-born son, Hunter, helped disabuse him of the magical thinking that was fogging his writing process. At some point, Biden said, he had mentioned to Hunter some words that Beau had spoken to him two weeks before he passed away. “And Hunter said, ‘Dad—Beau couldn’t speak for two months before he died! He had a tracheotomy!,’ ” Biden said. “I knew that. But I had put it out of my mind. I could not let myself think about my boy in pain.”

In Promise Me, Dad, Biden faces Beau’s trials head-on: the early uncertainty over what was ailing his son; the brutal diagnosis of the tumor as a glioblastoma (“the Monster,” as Biden’s own White House physician, Dr. Kevin O’Connor, called it); the hopeful period when Beau was responding well to treatment; the racking, last-ditch experimental procedures that Beau stoically endured after his symptoms took a turn for the worse; and, ultimately, the death of a man who was not only beloved within his close-knit family but also a political comer, a popular, charismatic figure in his native Delaware. From 2007 to 2015, Beau served as the state’s attorney general. He was also an officer in the Delaware Army National Guard and spent a year on active duty in Iraq. Before he got sick, Beau had planned to run for governor of Delaware in 2016. Given his impressive résumé and widespread appeal—Beau, his father writes, “had all the best of me, but with the bugs and flaws engineered out”—he might have gone farther still.