A few minutes into our conversation, bestselling author Holly Jackson is convinced she’s spotted the American singer Nick Jonas. It would be surprising, since we’re having coffee in Wimbledon. “It’s not,” she realises, “it’s just a lookalike.” Turning back to me, she says she has “gone off the Jonas Brothers, anyway” since Joe, formerly her favourite member, split from Game of Thrones star Sophie Turner.

The 32-year-old author is clearly more used to being the fangirl than the fangirled – despite the fact that her young adult series, A Good Girl’s Guide to Murder, has sold millions of copies worldwide, and is currently being made into a BBC TV series.

The writer has taken a break from working on her sixth novel to meet me near where she lives with her “boyf- I should say husband”, who works for a real estate company. The pair got married last September, but Jackson says it still “feels too weird” to call him her husband. “I don’t want people to know I’m old!”

She admits she has been doing some “real grownup stuff” over the last few years, such as getting a cocker spaniel and buying a house, which has allowed her to progress from writing books at the kitchen table to an office with “an actual proper desk”. When she wrote A Good Girl’s Guide to Murder she was still living with her parents in Amersham, Buckinghamshire, having moved back there after studying English at Nottingham. “Funnily enough, I did actually start with English and creative writing,” she says, but “ditched the creative writing after the first semester.”

The “forgotten middle child” of a BBC production assistant mother and a camera operator father, Jackson says she knew she wanted to be a writer from the age of 11, completing a 400-page fantasy novel as a 15-year-old which she “bullied [her] little sister into reading”.

The first manuscript she sent to agents when she was 25 was very different from the teenage murder mysteries for which she would become known. Jackson can see, looking back, why her adult novel “about a thief with obsessive compulsive disorder in a dystopian 15th/16th-century England” didn’t get picked up. It was “too many things”. When her now agent read the novel and asked if she had any other ideas, she proposed a story about a 16-year-old class swot who begins to solve a local murder for her A-level extended project. He liked the sound of it, so she went away and wrote A Good Girl’s Guide to Murder “in about seven weeks, because I didn’t want him to forget me”. It was the UK’s bestselling YA debut of 2019, and has continued to sell in huge volumes, along with its two sequels Good Girl, Bad Blood and As Good As Dead, and a standalone thriller, Five Survive.

Last year, Jackson not only sold more books than any other young adult author, she was also the top-selling female author of crime fiction in the UK. “Who’s above me?” she asks. It’s Richard Osman, James Patterson, and Lee and Andrew Child. “I will defeat them,” she remarks. “I think Richard Osman is the one I’ve got to go for. He’s very tall though.”

He does seem quite gentle, I note. “Yeah and I’m not,” she responds. “I would fight dirty, so I’ve got that going for me.” Joking aside, Jackson admits “it is wild” how successful her series has been. “I can’t really work out why it’s not dead yet. I mean, I’m not complaining about it!”

A big reason for its continued success is TikTok: Jackson’s books are some of the most recommended among the #BookTok community. The author says she “joined TikTok against [her] will”, because so many of her readers use it, but now finds the app so addictive that she has it blocked on her phone – it can be opened with a password that only her husband knows.

Jackson “can’t work out the alchemy” of why certain books become TikTok sensations. Sometimes, she’ll read a book because it has been hyped on the app and think: “This is the worst book I’ve ever read. What does everyone see in it?” She’s not into the current trend of “romantasy”. “I tried,” she says, but “I don’t like dragons and snogging.”

Thrillers have always been Jackson’s genre of choice: she remembers faking illness when she was 10 to skip school and watch her dad’s box set of 24, and reading Stephen King at an age when she “definitely shouldn’t have been”. A lifelong gamer, she also played Tomb Raider on her parents’ desktop in the “pre-PlayStation days”. In one Christmas family video she can be seen turning away from her game with a “goblin smile” and saying: “Look Daddy, I can snap her spine!”

“Maybe it’s lucky that I’m a crime writer, otherwise I’d be a serial killer,” she jokes. Yet playing video games “has probably been one of the biggest reasons that I wanted to be a storyteller,” she says. “Obviously, if you want to be a writer, you should read books, but I think you should definitely watch TV and play video games as well.”

Readers of her novels won’t be surprised to hear that Jackson is an “avid consumer” of true crime media, too – A Good Girl’s protagonist, Pip, has her own true crime podcast. True crime is a form of storytelling, too, Jackson points out, despite the fact that we tend to consume it as fact. “In things like Making a Murderer, you’re being told a story, you’re not actually being told the truth, because there are editors who are telling you exactly what they want you to believe,” she says.

This interest in the way true crime narratives are shaped by their creators was at the forefront of Jackson’s mind when she wrote her latest book, The Reappearance of Rachel Price. In it, Bel, a standoffish 18-year-old, has agreed to take part in a documentary about her mother, Rachel Price, who went missing from her home town of Gorham, New Hampshire, when her daughter was two. Not long after filming gets under way, Rachel – you guessed it – reappears, and Bel makes it her mission to work out whether Rachel is telling the truth about where she has been.

As in her previous books, the police don’t come across particularly well in this novel. Jackson thinks the modern justice system “often fails the people it’s supposedly meant to protect”. And, like Five Survive, The Reappearance of Rachel Price is set in the US. While this was partly because Jackson believes “America is a more fertile ground for crazy mystery thrillers” and means her characters can have “easy access to guns”, it was also a practical business decision. When the US rights for A Good Girl’s Guide to Murder were sold, the publisher stipulated that the American version of the book be set in the US. So Jackson had to “translate” the story to Connecticut, swapping references to the UK school and justice systems to the American equivalents, which was “quite arduous”. It was “partly a lazy decision” to set later books in the US, Jackson admits.

The author says she is ready “to try and branch out a bit”, and write a supernatural or paranormal mystery thriller. First, though, is the BBC adaptation of A Good Girl’s Guide to Murder. Jackson, who was “heavily involved” in the series’ production, admits to being nervous, but says she is “cautiously optimistic” that viewers will enjoy the show, especially given its cast, which includes Anna Maxwell Martin, Mathew Baynton and Emma Myers, who starred alongside Jenna Ortega in Netflix’s Wednesday. Jackson is back in full fangirl mode when I mention Myers. “Oh my gosh, she’s so good,” she says. “I’m so excited for everyone to see her as Pip, because she more than knocks it out of the park. She knocks it out of the Milky Way.”

Between promoting the BBC show and The Reappearance of Rachel Price, Jackson worries she won’t have much time this year to write her next novel, which is “very secret, hush hush”. “I’ll probably be told off by my agent for mentioning it at all,” she says, but “I’ll just mysteriously say that it might be a slight departure from what I’ve done before.”

I point out that, for someone so successful, she is very concerned about getting into trouble with her agent and publishers. “Well, that’s true,” she concedes. “If they tell me off, I will just write their names into a book as a horrible character.”

• The Reappearance of Rachel Price is published by Electric Monkey on Tuesday. To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply