Edna O’Brien’s new novel, her first in a decade, has already been hailed as “her masterpiece” by that master-of-them-all Philip Roth. And he’s right. This is a spectacular piece of work, massive and ferocious and far-reaching, yet also at times excruciatingly, almost unbearably, intimate. Holding you in its clutches from first page to last, it dares to address some of the darkest moral questions of our times while never once losing sight of the sliver of humanity at their core.

It begins arrestingly. A wanted Balkan war criminal, disguised as a self-styled “holistic healer” (he quickly drops the term “sex therapist” when he clocks the responses), fetches up in a little village on the west coast of Ireland. The parallels with the “butcher of Bosnia” Radovan Karadzic cannot be accidental, but neither is the novel’s power contingent upon them. With his “white beard and white hair tied up in a top-knot”, and his talk of herbs and tinctures and his constant “spouting” of Latin verses, “Doctor Vlad” sends a ripple of suspicion through the small community. Who exactly is this exotic stranger and what can he want?

The local garda has a half-hearted go at arresting him, only to be appeased with chat about football. A feisty nun, deciding that someone has to investigate this man, offers herself up for a hot stone massage - a scene that manages to be both enjoyably comic and queasily chilling at the same time. Most significantly of all, though, beautiful Fidelma, suffocating in a lonely, childless marriage, swiftly falls under the doctor’s spell and finds herself begging him to give her a baby. At first hesitant, he at last seems willing to oblige. A clandestine hotel room is booked.

And that’s just the beginning. To say too much about what happens next would spoil a truly gripping read. Suffice to say the hotel encounter will have consequences far darker and more startling (and more violent) than anything this reader had been able to imagine.

In fact, the novel turns out to be quite breathtaking in its twists and turns, its capacity to change shape and form and tone. As Fidelma leaves her homeland and moves through a myriad of different (yet all equally convincing and engaging) settings, the pace never falters. We find her cleaning offices along with other exploited migrant workers in central London, working at a home for retired greyhounds in the Kent countryside, and (unforgettably drawn, this one) attending a war crimes tribunal at the Hague.

Meanwhile, it’s impossible not to be knocked out by the sly perfection of O’Brien’s prose. Again and again and with apparent effortlessness, she changes tense (sometimes within a single chapter) or slides out of one character’s headspace and - with an absolutely convincing logic all of her own - into another. The effect is cumulative and the novel’s power creeps up on you; nothing is wasted, nothing forced. Devices that shouldn’t work - conveying lengthy, quasi documentary-style accounts of war through a dream, for instance, or even the dreaded long chunks of dialogue in a kind of foreign speak - work perfectly.

And O’Brien’s eye for detail is exemplary. All the quotidian detail of modern life, both rural and urban - even its omnipresent technology - is done with insight and wit. The garda’s mobile phone loses its signal in woodland. A woman at a book club walks around with her phone held aloft to show off her new-born foal. And as Fidelma and Vlad pick at their pre-consummation dinner, an elderly porter rushes to print out a leaflet illustrating the “mahogany newel posts” of the stairs they had earlier glancingly admired. Devices that shouldn’t work - conveying lengthy, quasi documentary-style accounts of war through a dream, for instance, or even the dreaded long chunks of dialogue in a kind of foreign speak - work perfectly.

Present-day London too: the B&Bs and charity organisations, the needless form-filling, the weary shift workers who are “night people, one step away from ghosts” and their harsh employers who are themselves exploited. All of this is conveyed with astonishing grit and clarity. Young people are every bit as nuanced and convincing as older ones: a beady six-year-old girl, glimpsed just a few times, inhabits the page with just as much weight and heft as a war-traumatised young waiter. And throughout it all, the presence of Doctor Vlad - alternately hot and cold, angry, cunning, charismatically reasonable and pitifully banal - somehow manages to infect every page.

The real genius of this novel - and I don’t use the word lightly - is to take us right up close to worlds that we normally only read about in newspapers, to make us sweat and care about them, and at the same time create something that feels utterly original, urgent, beautiful. It’s hard to believe that any novel could do more. And it’s hard - no, almost impossible - to believe that O’Brien is in her ninth decade, for this is absolutely the work of a writer in her prime and at the very height of her phenomenal powers.