Former president Barack Obama says in his upcoming book that the divisions in America "run deep" and the departure of Donald Trump from the White House will not be enough to bridge the divide.

In an excerpt from "A Promised Land," which goes on sale on Tuesday, Obama reflects on the four years since he left office.

"Our democracy seems to be teetering on the brink of crisis -- a crisis rooted in a fundamental contest between two opposing visions of what America is and what it should be," Obama said in the excerpt published on Thursday in The Atlantic.

The crisis "has left the body politic divided, angry, and mistrustful," he said.

It has also "allowed for an ongoing breach of institutional norms, procedural safeguards, and the adherence to basic facts that both Republicans and Democrats once took for granted."

Obama said he was "encouraged" by the election victory of his former vice president, Joe Biden, and his running mate Kamala Harris, and their "character and capacity to do what is right."

"But I also know that no single election will settle the matter," Obama said. "Our divisions run deep; our challenges are daunting."

"If I remain hopeful about the future, it's in large part because I've learned to place my faith in my fellow citizens, especially those of the next generation," he said.

"My book is for those young people -- an invitation to once again remake the world, and to bring about, through hard work, determination, and a big dose of imagination, an America that finally aligns with all that is best in us," Obama said.

In the 768-page first volume of his memoirs, the 44th US president reveals that he writes longhand with a pen on a yellow pad instead of on a computer and confesses to an inability to be concise.

He talks about trying to "find a discreet location to grab an evening smoke" while at the White House and how "drained, physically and emotionally" he and his wife, Michelle Obama, were when they left the White House in January 2017.

"For a month, Michelle and I slept late, ate leisurely dinners, went for long walks, swam in the ocean, took stock, replenished our friendship, rediscovered our love, and planned for a less eventful but hopefully no less satisfying second act," he said.