Isabel Colegate has just turned 90. It is a good innings for a good author, one with 14 books to her name. And yet Colegate’s work remains largely overlooked and under-appreciated, primarily because it is unavailable. For years, the majority of her books have languished out of print. Only her 1980 novel The Shooting Party has endured. In 2007, its status was confirmed and its posterity guaranteed when it was admitted to Penguin’s Modern Classics ranks.

Bloomsbury has taken tentative steps to open up Colegate’s back catalogue. Last year, the trilogy of novels that began with Orlando King (1968) was reissued in a single volume, while recently the 1964 book Statues in a Garden was rescued from obscurity. This might not constitute a full-scale literary revival but it’s still a commendable effort to restore Colegate’s fortunes and showcase her talents.

Colegate was a publisher’s assistant before she was a published author. After assisting authors with their careers, she got her own off the ground with The Blackmailer in 1958. It is a tightly plotted, elegantly written tale about a man on the make. Baldwin Reeves knows his superior officer in the Korean War, the late Anthony Lane, was not the valiant hero he is perceived as. Envious of the elevated social standing Lane had, Reeves decides to set the record straight and make a profit in the process by blackmailing his widow. With this debut Colegate started as she meant to go on, examining class, privilege and corruption often by way of high-born, well-connected, power-wielding characters.

She was no stranger to this breed – indeed, her father, Sir Arthur Colegate, was a Conservative MP, and both her parents came from families with country seats. Throughout her fiction she assumes the role of knowledgeable insider as well as aloof observer. It would be all too easy to carp, snark or lampoon these subjects from the sidelines. Instead Colegate reserves judgement and dispassionately records how the upper classes act and what makes them tick.

She does this brilliantly with Orlando King. The three books in the trilogy – Orlando King (1968), Orlando at the Brazen Threshold (1971) and Agatha (1973) – are deft updates of Sophocles’ Theban Plays, with Colegate’s titular hero a twentieth-century Oedipus Rex and his daughter Agatha as a reimagined Antigone. ‘We know the story of course,’ Colegate writes at the outset. ‘We are here profoundly to contemplate eternal truths.’

Orlando, the illegitimate son of two unmarried Cambridge undergraduates, is raised on a remote island off the coast of Brittany by a lecturer called King. In 1930, when Orlando turns 21, King sends him to England to make his mark. Mentors such as Leonard Gardner open doors for him but ultimately it is his own ruthless drive and ambition which propel him forward and enable him to secure power and command respect. He achieves success as both a businessman and a Conservative MP and for a while his future looks rosy. But one day he provokes Leonard into a blind rage which results in him driving off and dying behind the wheel of his car. It isn’t long before King is hit by the grim realisation that the man whose death he unwittingly brought about was in fact his biological father, and the woman he has taken as his lawful wedded wife is his stepmother.

The other two books in the trilogy follow Orlando into exile in Tuscany and track Agatha as she makes sense of her broken country and fractured family. All three books play out against seismic catastrophes which rocked Britain in the 1930s through to the early 1950s, from appeasement and world war to the Suez crisis and the Cambridge Five scandal. However, it is the first book which truly satisfies as a shrewd character study. As Colegate charts her antihero’s sharp rise and tragic downfall, she highlights his perceived decency and palpable magnetism while simultaneously laying bare his hypocrisies, betrayals and skewed morals. ‘Mr Orlando is a perfect gent,’ notes one character. ‘In other words a perfect shit.’

Statues in a Garden deals again with power play and ‘the hurly burly of politics’, and even incest (of sorts) raises its ugly head once more. This book’s events unfold over a much shorter timeframe, namely the summer of 1914. Sir Aylmer Weston, one of Asquith’s cabinet ministers, has much on his plate. At work he is preoccupied with the suffragettes’ campaign together with – what some characters almost cryptically term – ‘this Irish business’, ‘this Serbian affair’ and ‘the European situation’. He is also absorbed by preparations for his eldest daughter Violet’s wedding. What he is blissfully unaware of is the romance blossoming between his wife Cynthia and his adopted son Philip.

If this illicit relationship threatens the unity and the sanctity of the family, then so too does the imminent war. Cynthia, like every other member of the ‘minor gentry’ around her, is oblivious to the chaos that lies ahead. ‘We were born in a lucky age for people of our class,’ she declares. ‘We have such opportunities to be happy and do good.’ Philip, her wrecking-ball of a lover, will only be happy if great harm is done. He thinks a war with Ireland would be fun (‘We need a bit of blood letting’) and harbours a fervent desire to see his entitled family destroyed: ‘I want them to be blown to all the corners of the earth, and all their little trinkets lost.’ That destruction comes about sooner, and more mercilessly, than he could have imagined.

Colegate reprised this calm-before-the-storm setup and refined some of this novel’s tropes and themes in her masterpiece The Shooting Party. Set on Sir Randolph Nettleby’s Oxfordshire estate in autumn 1913, the book charts the eponymous event and its tragic outcome. It was adapted into a 1985 film starring James Mason and John Gielgud, and it inspired the likes of Gosford Park and Downton Abbey, not least through its rotating points of view and its range of characters from both the upstairs and downstairs quarters.

These characters are superbly well-defined. Sir Randolph has no truck with ‘imagination and all that rubbish’, firmly believing that ‘a man should stick to one country and be proud of it’ while fearing that his own is going to the dogs as landed proprietors such as himself are being stripped of power and possessions. His faithful wife Minnie cheerfully accepts life as ‘one long round of duty.’ Their guests flirt, gossip, bicker and keep up appearances despite simmering jealousies and resentments. Two characters stand out because they fall in love with one another. But will Olivia, stuck in a dull marriage to starchy Lord Lilburn, have the courage to break free and throw in her lot with Lionel?

In addition to the trigger-happy men and the accomplished women of leisure we meet the Nettlebys’ children along with the various members of staff who ensure the smooth running of their gilded lifestyle: John the footman; Ellen the maid; Glass the head-keeper; and Rogers the child-hating butler. As Colegate dips in and out of her characters’ affairs, her narrative takes the form of a series of tightly constructed episodes, some neatly linked, others standalone. Along the way she illuminates the manners, rituals and codes of this rarefied Edwardian world, explores gaping class divides, and in some cases shows superficially charmed lives to be hollow existences.

The book is also a chronicle of a death foretold. We learn on the first page that disaster will strike. We read on wondering who from this huge cast will be bumped off. Could it be the Nettlebys’ younger son Osbert, who wanders around distraught in search of his pet duck while men go about their ‘purposes of massacre’? Or will anti-blood sports intruder and protestor Cornelius Cardew become caught in the crossfire?

Colegate keeps us guessing and delivers the blow when we least expect it. But this individual death, though shocking, is soon dwarfed by wholesale carnage: ‘By the time the next season came round,’ writes Colegate, ‘a bigger shooting party had begun, in Flanders.’

This novel and others skilfully build to a calamitous change. ‘It sometimes seems as if events foreshadow themselves,’ Colegate wrote in her second novel, A Man of Power (1960), ‘or as if preliminary rumblings of circumstance precede a great upheaval.’ That upheaval often signals the end of something – an era, the elite class, the old order, or what Colegate has called ‘the last faint squeak of the British Empire.’ Randolph is adamant that an age, ‘even perhaps a civilisation, is coming to an end – it is happening all over Europe.’ Orlando King’s beleaguered wife Judith goes even further when contemplating impending war: ‘It’s the end of everything.’

Colegate once tried to throw light on her creative process: ‘People think novelists write fictionalised autobiography; perhaps some do. I think myself that the process by which one turns one’s deepest preoccupations into fiction is a lot more complicated than that – a kind of willed dreaming as necessary and as hard to analyse as the involuntary sort.’

That willed dreaming has worked wonders throughout a long and varied career. Colegate’s last book, her first full-length foray into nonfiction, was about hermits and solitaries. That was almost 20 years ago. Perhaps she is writing another book at her home in Somerset. Or perhaps she has laid down her pen. Whatever the case, she has produced an impressive body of work and deserves to be mentioned alongside other grandes dames of English letters such as Penelope Lively and Jane Gardam. Until more of that back catalogue sees the light of day, we should read, and relish, those few books we have.