As a teenage goth in Newcastle upon Tyne, Eliza Clark would fall down Wikipedia rabbit holes in her bedroom while tuning into lurid true-crime podcasts with an avidity she now calls “prurient”. The 29-year-old, recently named on Granta magazine’s prestigious once-a-decade roll of best young British novelists, suspects that was how she first encountered the horrifying case of Shanda Sharer, a US schoolgirl tortured and burned alive by four teenagers in 1992, two years before the author was born. It stuck: Clark’s new novel, Penance, styled as a true-crime story, centres on the grisly murder of a teenager set on fire by three of her schoolmates in a North Yorkshire seaside town on the night of the Brexit vote in 2016.

“I’m just interested in people’s lives and the histories of places,” Clark says, sitting down with me in a park near her south London home as a heatwave brews. “True crime, done well, feels like one of the only times you get to read nonfiction about day-to-day lives.” The “junk” podcasts she feasted on in her youth were a gateway in adulthood to the forensic heft of genre classics such as Brian Masters’s “She Must Have Known”: The Trial of Rosemary West. “He makes those crimes feel inevitable by stacking up the circumstances; so much of true crime is salacious – it isn’t interested in what makes [perpetrators] that way.”

Reading Masters put Clark in mind to concoct a murder narrative that would double as a sociopolitical excavation of small-town history. More intricate than her 2020 debut Boy Parts – a sleeper hit thanks to TikTok – the skin-prickling result is a pitch-dark mosaic of murky families and deadly frenemies, chronicling the slow decline of an ill-omened holiday resort with supernatural chills and metafictional twists; think The Secret Diary of Laura Palmer crossed with Mean Girls as told by Gordon Burn, if Burn – another Newcastle writer drawn to crime – had lived long enough to turn his laser-like gaze on the dank crevices of the web.

Adolescence is trauma, the novel seems to say; was it thus for Clark? “I didn’t set anyone on fire,” she says, deadpan behind leopard-print shades. Nor did she fall, like one character, into the online abyss of school-shooter fandom (not an invented subculture, in case you’re as sheltered as I am). “It’s just a horrible time, isn’t it? You behave and look worse than at any time in your life; but apart from all the hormonal stuff, your unstable sense of self combines with a lack of full understanding of consequence and permanence,” she says.

The three years Penance took to write were, she says, akin to pulling teeth, unlike the pleasure she got from Boy Parts, a mischievous satire narrated by a predatory photographer whose images of her male victims are hailed at a hip London gallery as edgy roleplay. “People who’ve read it maybe think I’ll be more of a wind-up merchant when they meet me, but I’ve got more of a primary school teacher energy than an enfant terrible vibe,” Clark says.

Written when she was 24, in eight months of weekends off from a day job at Newcastle’s Apple store, Boy Parts has so far sold 60,000 copies, she says: strong numbers for any literary debut, especially one from a tiny independent house such as north London’s Influx Press, which said yes to Clark’s cold pitch after she was snubbed by 12 agents. The book went more or less unreviewed – coming out in the plague summer of 2020 didn’t help – yet steadily amassed word-of-mouth buzz. About a year and a half after publication, Clark began to notice an extra digit on her royalty cheques. “It was TikTok. I don’t use it, so I had no idea. One of my friends said, it’s everywhere, there are videos about it that have hundreds and hundreds of thousands of views.”

Clark downplays the shock value of Boy Parts, calling it “baby’s first transgressive fiction”, and isn’t afraid to make clear that Penance’s broader canvas is a bid to be taken “more seriously”. She was recently invited to interview Bret Easton Ellis on stage, only for Ellis to veto the choice; too young, apparently. Surely her Granta listing brings recognition? Sort of, she says. Even at the magazine’s headline celebration event, Clark was repeatedly asked why she was there: “‘I’m on the list! My picture’s over there! It’s my special day!’ I feel quite lucky that I don’t have more of an ego – or, rather, that my expectations are just permanently on the floor,” she says.

She reckons that being a word-of-mouth success let her fly under the radar of critics ready to dismiss Granta’s selection as a list of commercially unsuccessful unknowns. “But you don’t want to be the arsehole who’s like: ‘Actually, if you were to check TikTok and ask 20-year-old girls on humanities courses at university, you’d find that one of these books is actually very popular,’” she says, with a winningly wicked laugh.

Being underestimated is something of a theme with Clark. In her early teens she read keenly, led by her parents to Tolkien, George RR Martin and Stephen King while finding Nabokov and Murakami on her own (“Ryū, not Haruki,” she adds quickly, as if to make certain I know that extreme horror is her jam, not pervy magic realism). The kind of pupil who once wowed English teachers by writing “pages and pages and pages”, she was blocked by her school from applying to study English at Oxbridge because she got C grades in GCSE maths and French. She then fell into boozing with pals (“it was very easy to underage drink in Newcastle”) and out of love with reading – or at least with books. “My brain had been so boiled by the internet by that point. I shouldn’t have been allowed to have my own laptop! Shock images were so clickable and findable. But I used to write loads of fan fiction and I wouldn’t have had all those years of writing practice.”

Instead of English, she studied art, first in Newcastle then in London. No good at drawing – or so she felt – and “too shy” (unlike the narrator of Boy Parts) to ask people to pose for photos, she found that what she most enjoyed was writing a dissertation on how Michel Foucault’s ideas of surveillance play out in the online era. By day, she sold posh undies at Agent Provocateur, having previously worked in bars. Returning home on graduation meant pulling pints again (“there’s not a lot of luxury retail where I’m from”), but this time she wasn’t able to blag a drink on shift – a perk she’d enjoyed in London – and the bouncers were useless: “I’d be dead sober, there’d be a man sexually harassing me and my manager would be like, ‘Well, he’s a paying customer.’”

Those experiences, visible in Boy Parts, made Clark crave a nine-to-five office job. Applying to local arts organisations led her to the writing development agency New Writing North, which encouraged her to try for its mentorship scheme; next came stints at Mslexia, the magazine for female writers, and the writing charity Arvon. Clark credits that CV with showing her how precarious and rejection-laden writing can be; it meant she entered the industry under no illusions. Yet her goal was always to write full-time and buy a flat – which made it a “no-brainer”, she says, to quit Influx for more money at her current publisher, Faber, despite her gratitude to them for giving Boy Parts a platform.

Now the Clark pipeline is running hot: as well as several screen projects she can’t discuss, she’s writing another novel (“a kind of speculative fiction thing”); in the autumn, there’s a stage adaptation of Boy Parts (which has also been optioned); and next year there will be a story collection “bouncing around” sci-fi and horror (one of the stories, She’s Always Hungry, is in the current issue of Granta; if you’ve read it and were left puzzled, Clark says 2,000 words were lopped off the end “in a way that may not be clear”, her admirably level phrase).

What was never on the cards was Boy Parts 2. Of Penance, Clark says: “I wanted to show I was a versatile writer with a career ahead of me; I’m not just some one-trick pony who wrote a weird little TikTok book.” Something in that mix of steel and self-deprecation tells me she won’t be underestimated for long.

• Penance is published by Faber on 6 July (£14.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply