I think you’d struggle to find a more-read author in the 1980s than Stephen King. Throughout the 70s and early 80s, King carved a path for himself as the world’s foremost writer of horror fiction. His books were a double threat: they were both immaculately written and presented, on their most basic levels, subjects that were designed to terrify readers. Each book focused on a topic that drilled into our own neuroses, either real or imagined: the vampires found in Salem’s Lot, the isolation presented by The Shining, the rabid dog that was Cujo. You could almost target the books to the fears of the individual: oh, you’re scared of cars? Well, have I got a horror novel to sell to you!

Then, in 1986, King wrote It. Suddenly, he wasn’t writing about the one thing that scared you; he was writing about everything that did.

It seems that King was taking lessons from himself when he wrote It. Indeed, he called the novel “the summation of everything I have learned and done in my whole life to this point”. Those references and nods are clear to King’s Constant Readers (the name he gives to his fans): for It’s main narrative thrust, it almost seemed as if he looked at the teenage friendships he’d written in 1982’s The Body (later adapted into film as Stand By Me) and took the basic setup of that book, deciding to push it further. What if there’s a group of friends who find something? What if it’s something they have to keep as a secret, something which deeply affects their lives – not just as children, but as adults as well? Fears aren’t something, King realised, that are confined to us as children. Those same terrifying thoughts often rear their heads in our adult lives, often manifesting in vastly different, far more complicated ways.

The novel is really one of two halves, two timelines that alternate throughout the novel. The first of these narratives takes place in 1957, when the central group of protagonists – the self-titled Losers’ Club, seven kids in their young teens – are dealing with life in the small town of Derry. In the aftermath of the loss of George, the six-year-old brother of Bill, the group’s de facto leader – the young boy’s arm torn off when he reached into a gutter to retrieve his paper boat – they all encounter the titular malevolent presence in one way or another.

Only, It doesn’t simply appear to the Losers’ Club in the same way each time: instead, it manifests as the thing that they’re most afraid of. For two of them, the fear is a stereotype, a Wolfman and a Mummy stolen straight from the classic Universal Pictures horror films of the 1940s. For the others, the fear gets more insidious: it takes the form of one of their own number; as a group of local boys who died where these kids play; and for Beverley, the group’s only girl, it comes in the form of torrents of blood (the same metaphorical horror as King once used in Carrie, his debut novel). Gradually, over the year, the Losers’ Club start to realise that there’s a pattern to deaths in the area; that It is a malevolence which recurs every 27 years or so, killing kids and then retreating until the next cycle.

At the same time, we meet up with the Losers’ Club in 1984. No playing with nostalgia here, no fears about growing up; this section deals firmly with the having grown, having lost what you once had. The death of youth rears its head large here, with the kids having grown up to be successful adults, but not necessarily happy ones. Their fears – the deeper fears, this is, rather than the more overtly visual horrors of their youth – haven’t faded. They’re still scared of being usurped; of being alone; of being forgotten; of the abuse they suffered as children. This last point is perhaps the most important, as it’s over the course of this section that the depth of the torments they suffered as kids is writ large enough for us to fully understand: this isn’t a novel about scary clowns and giant spiders (though both are there on the page), but about the depths of pain that teenagers suffer.

Tim Curry in the small-screen adaptation of It. Photograph: Allstar/Cinetext/Lorimar Television

Everything becomes about a cycle, and how hard cycles are to break. The town itself – it wasn’t the first time that King wrote about a town which was haunted by its own past, and it wouldn’t be the last – was trapped in the largest, but beyond that, each character was as well. Trapped in cycles of feeling useless; of unrequited love; of hate; of pain; of abuse. It’s the final one that rings loudest these days: seeing Beverley repeating her own cycles of abuse in her adult life is truly devastating.

The novel’s ending – which I won’t spoil here – relies on something that feels obvious from page one; that the Losers’ Club are going to have to try to remember what it was like to be kids – to be naive and innocent, to have fears be something that feels as though they can be overcome – in order to finally defeat It. As King wrote, he “sees children as either victims or forces of good” – it’s the adults who ruin things in his books. Recapturing that goodness is a theme that rings through readings of the novel – and it’s a novel I’ve read a fair few times – and perhaps the only theme that really matters.

I first read the book as a teenager, the same age as the Losers’ Club were; and now I’m the age they were when It came back to Derry. I can see their terrors of growing old, their worries about their failures, about what’s been lost to them thanks to the passage of time; and I can see them just as clearly as King’s protagonists could – as, I’m sure, King did himself. I’m not scared of the same things that scared me when I was a kid; at least, not on the surface. But dig deeper, as King did with his Losers’ Club, and those fears are absolutely still present.

That’s why, in the 30 years since publication, the public’s obsession with It hasn’t really waned. We’re obsessed because we all have fears. We all have things that scare us, be that the aforementioned clowns and spiders (and really, thanks Tim Curry for ensuring that an entire generation of children who like to watch horror movie adaptations of their favourite novels will be scarred for life), or things that lurk far deeper in our psyches. This book speaks to everybody. It’s King’s scariest novel, and I doubt that will ever change.

And there’s a new movie adaption of It due to hit cinemas in 2017, with a new cast of kids (including Finn Wolfhard, last seen in the King-tribute Netflix show Stranger Things) and a brand new Pennywise, all designed to scare a new generation witless. It’s been 30 years since the release of the novel, and 27 years since Pennywise hit TV screens. It is, as the lore of the book would have it, about time he came back.