On 14 April 1865, less than a week after Confederate General Robert E Lee surrendered his army in Virginia and effectively ended the American Civil War, John Wilkes Booth gained entry to the private box at the theatre in Washington, DC, where Abraham Lincoln and his guests were watching a performance of Our American Cousin and shot the president in the head. Lincoln was pronounced dead the next morning. His assassination thrust much of the country into fresh despair and prompted the largest manhunt in American history.

Booth is Karen Joy Fowler’s ambitious exploration of the family at the centre of this seismic moment in the making of modern America – the entire family, that is, except John Wilkes Booth himself. In an author’s note at the end of the novel, Fowler acknowledges that it was never her intention to write a book about John Wilkes (known to the family as Johnny). He was, she writes, ‘a man who craved attention and has gotten too much of it; I didn’t think he deserved mine.’ What interested her was not the motives or the mindset of a violent murderer. It was the impact of that murderer’s crime on those closest to him, the people who had loved him all their lives.

Fowler has tackled this question before. Not one but two short stories in her 2010 collection, What I Didn’t See, consider Lincoln’s assassination from the sidelines: ‘Standing Room Only’ from the giddily infatuated perspective of the daughter of John Wilkes’s landlady and ‘Booth’s Ghost’ through the eyes of his appalled older brother Edwin. In both stories, the ghosts of the dead haunt the narrative. The ghosts are a presence in Booth too, but this time Fowler extends her reach across decades and generations, constructing her partial (in both sense) family history from the perspectives of Johnny’s three closest siblings: successful, tormented Edwin; clever, inhibited Rosalie; and, Johnny’s favourite, the beautiful, self-absorbed, adoring Asia.

Long before the assassination of Lincoln, the Booth family was famous across America. The patriarch, Junius Brutus Booth, was already a leading light in English theatre when he fled to Maryland in 1821 with his mistress. He came to be regarded as one of the great Shakespeareans of the American stage, celebrated for his towering Richard III. Of his six surviving children, three followed Junius into the theatre; by mid-century Edwin’s reputation would rival his father’s.

Fowler’s story unfolds against a backdrop of escalating national tension; she captures with enthralling vividness a country caught in the grip of fanatical populism, ripped apart by irreconcilable political differences and boiling with fury and rage. Her greater achievement, though, is to breathe life into this turbulent, eccentric, bohemian family, investing each of its members with an intense, and intensely affecting, humanity.

This should come as no surprise. Fowler has always combined a forensic insight into human nature with deep compassion and, delightfully, a wicked sense of humour. Her last novel, We Are All Completely Beside Ourselves, which revolves around a 1970s experiment in New York involving the raising of a baby chimpanzee in a human family, contrived in turns to be charming, provocative, heartbreaking, and laugh-out-loud funny. The book won the 2014 PEN/Faulkner Prize and was shortlisted for the Booker Prize.

While Booth is superficially a very different kind of book, epic in both scope and ambition, it sees Fowler returning to many of the same preoccupations. At one point in We Are All Completely Beside Ourselves, a fellow student describes her family to Rosemary, the tight-lipped protagonist. ‘Like the rest of the family is fine, you know?’ she says. ‘And then this one crazy sister goes and ruins it all.’

Swap ‘brother’ for ‘sister’ and she might be describing the Booths: there can be no doubt that crazy Johnny ruined everything. The question at the heart of this outstanding novel is whether the rest of the family was ever fine. Junius Booth was an unorthodox father who raised his children as free-thinking vegetarians. As a self-appointed Southerner and die-hard supporter of slavery and secession, the rabidly Confederate Johnny was always going to find himself at odds with his progressive-minded siblings. And yet the other Booths were far from perfect. Volatile, quarrelsome, plagued by alcoholism and ill health, the family was repeatedly mired in scandal and debt. Fowler takes this potent brew and, from it, distils a portrait of a nation in crisis and of a family struggling to survive, a family so caught up in the ordinary business of living and trying to love one another that they not only fail to avert the terrible crisis that will engulf them but fail to see it coming at all.

The result is an unalloyed triumph. Like Hilary Mantel in her Wolf Hall trilogy, Fowler skilfully exploits the present tense to remake the past as something surprising and new. A year on from the insurrection at the US Capitol, as the mainstream media in America continues to speculate about a second civil war, the parallels with today’s post-Trump landscape are unavoidable. But it is the Booth family that lingers when the book is closed, each member completely themselves as, fearfully sometimes and sometimes with reckless abandon, they fumble their way towards an unimaginable future.