This is a presidential memoir that was perfectly timed, even though it was reportedly a year late.

If A Promised Land had landed in 2019, it would have had an undue influence on the dynamics of choosing a candidate to take down Donald Trump, the man who succeeded Obama four years ago, unleashing a persona and agenda that was deeply destructive of the tenets and norms of America’s democracy.

If Trump had won a second term, if his steps to overturn and nullify November's election had been successful, Trump would have used these next four years to destroy the pillars of American democracy. Have no doubt about that: with 70 million voters behind him, and with 50 per cent or more of Republicans believing Trump won the election, 1776 would have had to be reinvented.

Early on, with Michelle, he tells her he wants to enter politics to do what is right: “What’s the point of having a fancy law degree if you can't take some risks?”

Obama’s writing, by showing us how well he knows himself, reminds us of how exceptional a man he is. He does not have many visible warts, but in transcending himself he allows us to see them.

Early on, with Michelle, he tells her he wants to enter politics to do what is right: “What’s the point of having a fancy law degree if you can't take some risks?”

She replies: “Have you ever noticed that if there is a hard way and an easy way, you choose the hard way every time? Why do you think that is?”

Obama wonders whether his political ambition is akin to the addiction of an alcoholic: “Was it just vanity? Or perhaps something darker - a raw hunger, a blind ambition wrapped in the gauzy language of service? If one of the qualifications of running for the most powerful office in the world was megalomania, it appeared I was passing the test.”

Michelle kept asking him to ask himself, “Why me?” and Obama said that the world will look at America differently, and Black kids and Hispanic kids would see themselves differently too, “and that alone would be worth it”. Julia Gillard, Jacinda Ardern and Kamala Harris have all said the same thing.

Unlike the outgoing president, who would ask “Why did it take so long?” if he was ever awarded the Nobel Peace Prize (reality check: he will never receive it), Obama was grounded enough that when told at 6am one October morning in 2009 that he had won it, he immediately asked “For what?”. He told Michelle. “That's wonderful, honey,” she said, and rolled over for more sleep.

Obama not only knew when to run, and why he was running, but he came to understand as president when to act and do the job of being president. A week after the assassination of John F. Kennedy, Lyndon Johnson was meeting with his top aides to map out their priorities for the year ahead. Johnson wanted to do civil rights; it was his highest priority. His staff were terribly afraid it would cost him re-election. “What the hell is the presidency for?” Johnson thundered. And he got the civil rights bill – and much more – done.

Obama's signature legislative achievement was universal access to affordable health care (Disclosure: I worked on the staff in Congress with the chairman, Henry Waxman, who got that bill into law.)

Towards the end of his first year in office, Obamacare was deadlocked. The senior staff told Obama to cut a deal for half a loaf and leave it: “You won’t get everything you wanted, but it will still help a lot of people and we can make progress on the rest of your agenda.”

Phil Schiliro, Obama’s chief legislative strategist, saw a way to get it done, and asked Obama: “Do you feel lucky?”

“Where are we, Phil?” Obama answered.

“The Oval Office?”

“And what’s my name?”

“Barack Obama.”

“Barack Hussein Obama. And I'm here with you in the Oval Office. Brother, I always feel lucky.”

And Obamacare did become law. Obama writes: “I often felt steadiest when things were going to hell.”

Obama's book did not come out in time to be a factor in the Democratic primaries, as voters were weighing Joe Biden’s candidacy. We can now see, by virtue of the kind of vice-president Biden was, that he has the ability and judgment to do the job.

And Osama Bin Laden was killed, partially redeeming the terror he visited on the United States. Opinion was divided on whether to carry out the strike. Obama knew failure could end his presidency – just as the Iran hostage crisis helped make Jimmy Carter a one-term president in 1980. But he trusted the decision-making process and the intelligence, and Bin Laden was removed from this earth.

Obama's book did not come out in time to be a factor in the Democratic primaries, as voters were weighing Joe Biden’s candidacy. We can now see, by virtue of the kind of vice-president Biden was, that he has the ability and judgment to do the job.

Obama chose Biden to be VP because even though he liked the limelight too much sometimes, and wasn’t as self-aware as he could be, Obama was certain Joe would be “more than ready” to serve as president if something happened to Obama, that his foreign policy experience was needed by the younger president and that his ties in Congress would be invaluable. “What mattered most, though, was what my gut told me – that Joe was decent, honest and loyal. I believed that he cared about ordinary people, and that when things got tough, I could trust him. I wouldn't be disappointed.”

Biden got the Republican votes needed to pass the economic recovery bill to combat the Great Recession, and he made sure the programs delivered their intended benefits for the American people. Biden took on the generals on Afghanistan policy and had Obama’s back on foreign policy.

We are about to see Obama’s judgment on Biden played out for all of us, in America and around the world.

The most haunting, agonising question about the past four years is: How the hell did this happen? How, why did Trump follow Obama? How could that massive, teary wave of goodwill and pride of tens of millions of Americans across the country that November election night in 2008, that they had chosen a black man as president as an affirmation of what America is – how could that be followed eight years later by Trump, whose America First agenda and inauguration day invocation of “American Carnage” heralded leadership that wilfully divided and tore apart the United States? How could this be?

Obama discerns the emergence on the landscape of what would become Trumpism with the selection of Sarah Palin, the governor of Alaska, as Senator John McCain’s running mate in that 2008 election. We know Palin could see Russia from her front porch and was the pin cushion on Saturday Night Live, and that she has faded into oblivion.

But what Obama saw was “a potent disruptor” with “a biography tailor-made for working-class white voters who hated Washington and harboured the not entirely unjustified suspicion that big-city elites looked down on their way of life”. He also realised that her media-savvy moxie through “Fox News, talk radio and the budding power of social media” could rocket her into her audience. “In a different time and a different place ... the sheer energy Palin generated within the Republican base might have had me worried.”

Obama writes that “our democracy seems to be teetering on the brink of crisis … a crisis that has left the body politic divided, angry and mistrustful, and has allowed for an ongoing breach of institutional norms, procedural safeguards and the adherence to basic facts that both Democrats and Republicans once took for granted”.

Obama never faced it again. But that different time did arrive. Hillary Clinton paid the price for referring to the Trump base as “deplorables”. And Trump ruled for four years.

Another Trump driver is that Obama’s legislative achievements worked too slowly. Obamacare was not hugely popular until Trump tried to repeal it. Despite a decade of growth, the economic recovery program took years to get employment back to normal levels. The rap on Obama was that he had tried to do too much at the expense of focusing like a laser on the economy - that he was just another big-spending liberal with a climate change agenda that raised energy prices. Two years in, it cost him his majority in the House of Representatives.

Obama knew that he had “failed to rally the nation, as FDR had once done, behind what I knew was right”. And the Right was unforgiving: hyper-partisanship was evident from day one of the Obama presidency. Unrelenting criticism together with fringe elements of white supremacy entered the mainstream.

With Trump, what engaged the country was not just his flagship issues of nativism, isolationism, protectionism and nationalism. No, the theatrical element was paramount too, as he took over cable news screens with “news” stunts fuelled at that time with his lie that Obama was not born in the United States.

“What I knew,” Obama writes, “was that Trump was a spectacle, and in the United States of America in 2011, that was a form of power. Trump trafficked in a currency that, however shallow, seemed to gain more purchase with each passing day.” He got immense coverage and airtime. “Far from being ostracised for the conspiracies he peddled, he in fact had never been bigger.”

We will see, sooner rather than later, if we are in the post-Trump era. And that is not just up to Joe Biden.

Obama writes that “our democracy seems to be teetering on the brink of crisis … a crisis that has left the body politic divided, angry and mistrustful, and has allowed for an ongoing breach of institutional norms, procedural safeguards and the adherence to basic facts that both Democrats and Republicans once took for granted”.

Pretty basic stuff. But the chasm is enormous.

Or is it? “But the idea of America, the promise of America: this I cling to with a stubbornness that surprised even me. ‘We hold these truths to be self-evident, that all men are created equal’ - that was my America.” And Obama was, and is, true to it.

Many wonder what is next for Obama. Secretary General of the UN? Chief Justice of the Supreme Court? No. I think Obama is in the process of becoming America's Nelson Mandela, who served as South Africa's president after winning the struggle for freedom by calling his country to higher ideals. In that same spirit, Obama insists America keep faith with its highest principles, to never deviate from the journey that bends the moral arc of the universe towards justice, so that ultimately we not only see it from the mountain, but we do arrive and celebrate and cherish, together, a promised land.