The creation of an original fictional world, separate from or at least adjacent to our own, remains one of the most demanding challenges that is available to the writer of fiction. No matter how outlandish or bizarre an alternative reality may be, it must, at least if the reader is to accept it without question, feel plausible and grounded. The rules that govern its operation may be very different from those that affect our own lives, but they should feel palpable and consistent. The most enduring of these creations – Tolkien’s Middle Earth, Swift’s land of the Houyhnhnms, Lyra’s world in Philip Pullman’s Northern Lights, say – can come to feel compellingly real. Less successful attempts can appear merely hollow and unpersuasive, the act of imagination, however effortful on the part of the creator, proving insufficient to transport us.

Well over a decade ago, Susanna Clarke pulled off something close to this trick in Jonathan Strange & Mr Norrell, her long novel of an alternative past, which showed us an England filled with literal magic. Rich in detail and outré incident, it nonetheless never quite provided a story that had real momentum in its own right; the backdrop was ultimately more interesting than the drama.

Her new novel, Piranesi, is quite different. All of the strengths of Clarke’s writing are to be found here, and none of its flaws. The world that she depicts in this surprisingly slim book is highly distinctive yet altogether believable. At the outset we find ourselves, without explanation, in an impossibly vast house, occupied by wild animals and statues of the most curious sort, and with an ocean surging in the basement levels. The sole human figure in this desolate but rather beautiful place is Piranesi himself, who appears at first to be an odd, simple soul. The narration is his, written in a naive style: ‘Since the World began it is certain that there have existed fifteen people. Possibly there have been more; but I am a scientist and must proceed according to the evidence.’

The sheer weirdness of this setting and protagonist is enough at first to lead us into a labyrinth, but very soon the mystery deepens. We learn that another human, whom Piranesi calls ‘The Other’, sometimes visits these halls and that they contain several skeletons and relics suggestive of a world beyond: ‘At one time I was forever finding crisp packets scattered about the First Vestibule. I also found old fish finger packets and sausage-roll wrappings.’

To give away any more of this wholly satisfying plot would be to dilute its pleasures. I would recommend reading the book while knowing as little as possible about it (and, naturally enough, avoiding any other reviews) so as to let the pieces of the puzzle fall most effectively into place. It’s a subtle, deceptively complex work, though Clarke isn’t afraid of the occasional melodramatic flourish. Its influences are disparate and various – everything from the early short fiction of Will Self to David Mitchell’s Slade House to the more eccentric works of Colin Wilson – but it feels stylishly idiosyncratic and unique. Here is Clarke’s talent in full flower; Piranesi is the most purely enjoyable novel I’ve read in a long while.

Alex Pheby’s dense fourth novel, Mordew, promises to be the first in a trilogy. This will certainly be needed in order to explore completely the world that the author has created in this initial instalment. His hero, Nathan Treeves, is a young boy living in the slums of Mordew, a sprawling, steampunk-tinged coastal city, with his dying father and desperate mother. It is a harsh, improbable world that blends some of the tropes of young-adult fiction with both grim realism (Nathan’s mother has become a sex worker in order to keep a roof over the family’s head) and the kind of grisly fantasy at which Mervyn Peake excelled. There is a sense throughout of vivid squalor – ‘now, in the corners, rats and dead-life encroached on the shadows and the idea of happiness seemed nonsensical’ – and Pheby has an ear for ghoulish description: an adolescent possesses a voice that is ‘freshly broken, rattling like a beetle in a matchbox’, while a child being beaten thrashes ‘like a newly gelded weasel’.

Whereas Clarke tells her story almost entirely through a single first-person voice, Pheby’s story is told from a third-person perspective in a knowingly antique voice, the narrator sometimes dropping into the story to elucidate or offer a tangential explanation. The book features pages of supplementary material, including at the outset a long list of things to expect (‘an insect with the face of a monkey … delicious lollipops that come from nowhere … men with gills, but no eyes’) and no fewer than ninety pages of glossary, explaining in detail the many oddities of this place: ‘Limb-Baby (-ies) … Litter-Bearers … (The) Living Mud’. This weight of material can come to feel excessive, even exhausting – the fantasy equivalent of a historical novelist insisting on using up as much of their research as possible. The story that eventually emerges from this morass of information is intriguing, though, blending Great Expectations with Gormenghast, while the cliffhanger ending seems to hint at a more dynamic sequel.

Both of the invented universes here have the potential to find an appreciative readership. The deft construction at the heart of Piranesi will stay with me the longest. The novel also provides a useful lesson: that giving glimpses of some strange new realm can prove more potent and beguiling than the presentation to the reader of the whole of that world.