Twenty years since Harry Potter and the Philosopher's Stone and you will find the impact of the books in so many places it's hard to know where to begin. But let's start with the booksellers. Not the shops. The people.

Wander into a bookshop, aim for the brightest corner of the shop and find the children’s bookseller. Chances are they’ll be a helpful, patient, enthusiastic, expert 20something. They’ll have an encyclopaedic knowledge of the books surrounding them, plus the ones on the way next week, and you will leave with three more books than you intended to buy.

And the likelihood is the bookseller will be a child of Harry Potter.

Sure, there have always been obsessive young readers – the sort who read and walk at the same time, their parents pulling them aside before they collide with a lamppost. But more often than not, these current booksellers’ love of stories was formed in the white hot years during which Harry Potter became an unprecedented force on a generation. And how those readers responded changed everything.

Dividing line

There is a before and an after with children’s books. Harry Potter is the dividing line. There was great children’s writing before Harry Potter, and fine film and TV adaptations too, but on the whole kids’ books stayed where they belonged, in its section away from the proper books, until kids read their way to the end of the shelf and moved on to what appealed to them among the “grown-up” books.

You certainly didn't see kids' books in the hands of adults unless it was bedtime and their six-year-old wanted one more chapter before sleep. From Harry Potter and the Philosopher's Stone in 1997 up to 2007's Harry Potter and the Deathly Hallows, that idea went out the window.

Through a series whose themes matured with its readership, young readers grew into adulthood alongside its characters (in print and, later, on film). It closed the chasm between young adults and (supposed) proper ones, until everyone could read children’s literature unapologetically.

Children’s literature took off and has not yet stopped climbing. It now forms a quarter of the UK market, for instance, where it was 15 per cent in 2001. The preference for printed books made it a hold-out against digital, while the thickness of novels for young readers more than doubled in an age when attention spans were supposed to be shrinking. Now young readers have riches to pick from. Publishers compete for the riches in young readers.

In some ways, Harry Potter came out of nowhere. Whatever about Rowling’s famous early path – a single mother writing in a cafe while her daughter slept; the subsequent rejection letters – it took a couple of books into the series, and solid publicity graft, before things picked up. After which they continued to pick up. And kept going and going and show no signs whatsoever of stopping.

The amusement parks, the stage show, the movie franchises, the annual celebration night in bookshops, the various editions still holding a residency in the bestsellers lists, the massive boost to an entire industry. No one could ever have imagined this at the turn of the century. Harry Potter was meant to defeat Voldemort, but no one expected him to save reading.

‘Star Wars’

And yet, Harry Potter's effect is not entirely in isolation. Perhaps Star Wars – 40 years old last month – is its clearest comparison in how it seeped out of its original medium and into every corner of modern culture. Both Star Wars and Harry Potter are popular enough that even people who have never seen a frame or read a word know their characters and lexicon. But while Harry Potter released adults to read kids' books, through Star Wars the world had already past the point where children's entertainment was a huge driver of the showbusiness economy.

Comic books were at it too, their superheroes becoming the dominant genre of modern cinema. Neither is it a coincidence that while Rowling was writing the Harry Potter books, Pixar changed the nature of children's cinema and expectations of it. The likes of Toy Story and The Incredibles weren't just sophisticated, philosophical, funny, thrilling movies which rewarded repeated viewings, they cemented an idea that children's entertainment could not only be excellent, but that it should be.

That standard elevated children's television, where you could stumble upon Cartoon Network and – through the likes of Adventure Time and The Amazing World of Gumball – some of the most imaginative, wonderful and underrated television being made anywhere.

So, Harry Potter is a big part of a broader trend in which children’s culture became valued and valuable. Yet what it did for the books business alone is unique.

There have been phenomena before, and will be again; books so popular they lead to a goldrush of similar titles, cashing in before the tide goes out on their genre and some other craze washes up in its place.

The Da Vinci Code. The Girl on the Train. Even Angela's Ashes can be blamed for that once overwhelming genre of "misery lit".

Last year, there was the sudden appearance of adult colouring books. And yes, you can probably blame Harry Potter for giving licence to the idea that adults can buy colouring books.

There was Fifty Shades of Grey, which itself was originally Twilight fan fiction. Twilight was a Young Adult series which gained success in the wake of … well, you see the connection.

But none of those changed an industry permanently.

During and after the Harry Potter series, the children’s market grew, even as adult sales struggled. It meant that in shops, the children’s book sections began to creep forward towards the front until entire window displays were being given over to them. It meant roles for young fans to become passionate booksellers.

It led to a great increase in children's books, more prizes, more shops doing children's books of the month. There are more in-store events, cosplay events, launches. Easons developed a "Dept 51" annex for its teens and young adult section and won a major industry award for it.

David Walliams

It is not Rowling. While there has been a lot more fantasy writing in the wake of Harry Potter, and much of it in the form of series, she is far from the only influence. The extraordinarily successful David Walliams, to take just one, is a post-Roald Dahl writer, and you don't need to stand at children's bookshelves long to see how Dahl still looms large among those writer's ideas and styles. (To add some layers, most comedians now with book deals might be considered post-Walliams writers. If you meet a children's author at a party and want to get away quickly, don't mention comedians with book deals.)

With the rise in children’s books has come the competition and the demands. Book festivals give more and more time to children’s writers, who in turn must give more and more time to promotional events and school visits. A writer who only writes is an oddity these days.

At the recent Hay Festival in Wales, on its dedicated schoolchildren days, you could wander from tent to tent, finding children's authors performing (and it is performance) to audiences sizes all the way up to 1,700. Meet a children's writer at a festival now and they might be carrying a musical instrument or a case filled with props. They'll have visuals, jokes and a routine that has been honed over event after event.

JK Rowling stepped out of these publicity demands, because she didn’t need it. But, trust me, the rest of us most definitely do.

Children's writers are, then, working in a post-Potter world. Although, of course, there is nothing "post" about it. Rowling keeps creating, the plans for five Fantastic Beasts films expanding the Potterverse even if the last Harry Potter novel is now a decade old. She co-authored 2016's bestselling book in the UK and US (the play script for Harry Potter and the Cursed Child). The play recently won a record number of Olivier awards. There are new covers for the original series, and the books do not drop from the bestsellers.

At some point, JK Rowling will no longer be the single most important and constant force in the Potterverse. Some day, the Potter avalanche will abate. Surely. Maybe. Yet, you'll find Sherlock Holmes enduring and thriving in novels, movies, TV dramas 130 years after his first appearance. There is Batman, Superman, Spider-Man, X-Men. There's James Bond. Doctor Who…

So perhaps 20 years on from the first book, we may yet only be at the beginning of it all. Where exactly it goes is unknown. We can only be sure there is no going back.

Shane Hegarty is the author of the Darkmouth series, which are also being adapted for the big screen.