Robert Harris’s background in journalism is always evident in his books. The novels themselves are remarkably various, taking us from ancient Rome (his Cicero trilogy) to high finance (2011’s The Fear Index), from an alternative history of the second world war (Fatherland) to the hunt for the men who signed Charles I’s death warrant (Act of Oblivion). He’s a writer whose work is energised by its engagement with facts, but particularly those facts that have been hidden or occluded, requiring the twitching nose of the journalist to sniff them out.

Precipice is set in the summer of 1914: “that improbably glorious summer” before the great war opened up a chasm in the world. Harris was given access to an archive of letters, telegrams and official documents in the possession of the Bonham-Carter family, many of which are reprinted in the novel, several for the first time. Through these documents, he has constructed a quite brilliant novel about a clandestine love affair.

We first meet the Honourable Venetia Stanley, daughter of Lord and Lady Sheffield, in the buildup to a tragedy. She is part of a louche and aristocratic set, the Coterie, known for their cynicism and excess. Venetia decides not to join a cruise on the Thames during which one of their members, Sir Denis Anson, drowns. Venetia has other things on her mind. Although only 26, she’s in the midst of a romantic entanglement with the 61-year old Liberal prime minister, Herbert Henry Asquith.

Just as in his previous novel, Act of Oblivion, Harris has left the framework of history intact, his only invention being the character of a detective to act as the envoy of writer and reader. Here policeman Paul Deemer is initially called in to investigate the drowning on the pleasure cruise and then, elevated to Special Branch, to look into who in the cabinet might be leaking secret documents – a series of sensitive telegrams have been found scattered around the roads of the home counties.

The letters from Asquith to Venetia were written in the white heat of political life, with Ireland on the verge of revolution and war glowering on the horizon. This backdrop makes their content all the more jaw-dropping. They intermix the moonings of a love-sick teenager with spectacularly indiscreet revelations about the inner workings of government. Venetia’s letters to Asquith have been lost, and so Harris only has one side of the conversation. The novel’s brilliance lies in the way the author has written into the void, giving life and voice to Venetia, bringing her to dazzling life through her imagined letters to Asquith and Harris’s portrait of a bright, unconventional and complex young woman seeking to escape the strictures of her aristocratic upbringing.

The question that Harris has to wrestle with is not why the prime minister fell for Venetia – it’s clear from his letters that he is besotted – but rather why she should choose to pursue the relationship with the florid, white-haired Asquith. “She liked him for his cleverness, his fame and power which he wore lightly,” Harris writes. “She also enjoyed the thrill of it – the secrecy, the illicitness, the risk.” Asquith realises that in order to maintain her interest in him (and her willingness to join him on drives into the countryside, the blinds of the car drawn), he must feed her juicier and juicier material. As war breaks out and political wrangling over military strategy commences, we read the explosive missives from Asquith through the eyes of our detective, Paul Deemer. Suspicion begins to fall on Venetia’s German-speaking maid, Edith. The net around Asquith himself tightens.

This is a novel about love and power, about the son of “dour, Northern nonconformists” risen to the highest seat in the land. It’s interesting that there is no sense that Harris seeks to censure the older man for what might, in the light of modern ideas about power dynamics, be seen as a predatory relationship with a woman young enough to be his daughter. Rather, Asquith is presented with great tenderness and sympathy, as is his hard-edged but vulnerable wife, Margot. It’s the character of Venetia, though, that turns Precipice from a very good novel into a great one. As the horrors of the trenches are revealed, she feels called upon to play her part in the war effort, but also begins to question her relationship with an increasingly desperate, doddering and lovesick prime minister. At the time when our minds are on another global leader overtaken by age, the ending feels particularly poignant, its message nuanced yet terribly moving.

• Precipice by Robert Harris is published by Cornerstone (£22). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply

• This article was amended on 1 September 2024 because an earlier version referred to the book Act of Oblivion as concerning the hunt for the men who signed Charles II’s death warrant. That should have said Charles I’s death warrant.