In 2011, the prolific British writer Anthony Horowitz told an interviewer that Scorpia Rising, the ninth book in his bestselling series about teen spy Alex Rider, was “without question, the last book.” Now, nobody is as surprised as Horowitz that he must correct the record: the cover of the 10th book in the series, Never Say Die (Philomel, Oct.), is revealed here for the first time.

“I was polishing some short stories that were going to be published as a collection and as I was polishing them, I started rewriting them, and I rediscovered the character,” Horowitz said. “Honestly, I didn’t know I had another Alex book in me. It just popped out.”

The series, which launched in 2000 with Stormbreaker, has sold more than five million copies in the U.S., and an estimated 19 million copies worldwide. “Alex is still very popular with teachers and librarians, and kids were asking me, ‘What happened next?’ so it’s a bit driven by demand,” Horowitz said.

Though it’s been six years since readers last saw Alex, he’s aged only a few months since what had been billed as his concluding appearance. The new story takes place as he’s on his way to San Francisco, where the 15-year-old plans to live with his girlfriend’s parents and attend high school. Horowitz, who visited San Francisco earlier this year, posted a photo of himself “outside Alex’s home” on Twitter.

Alas, a normal life is not in the cards for Alex. A cryptic e-mail leads him to believe his former guardian, Jack Starbright, believed dead at the conclusion of Scorpia Rising, may still be alive. Alex, who wasn’t enjoying life as a high school student anyway, jets off to Egypt in search of answers.

Philomel plans to bring Horowitz to the U.S. in October for a tour in support of the new book. He is curious about who may show up in the audience. “If Twitter is any guide, then I would say the books are not just for young readers anymore,” Horowitz said. “I have a lot of followers who tell me they read the books when they were children and are still re-reading them now, people who are the age where they are getting married and buying their first homes.”

During Alex’s hiatus, his creator—once dubbed “the busiest writer in Britain” by the U.K. press—had no time to miss him. Horowitz was commissioned by the estate of Arthur Conan Doyle to write two new Sherlock Holmes novels (The House of Silk and Moriarty) and by the Ian Fleming estate to write a new James Bond novel, Trigger Mortis, which incorporated unpublished material written by Fleming. This June, HarperCollins will publish a new stand-alone novel, Magpie Murders, which Horowitz is also adapting as a screenplay.

Horowitz spent much of the past two decades writing the screenplays for the World War II television drama, Foyle’s War, which can now be watched in its entirety on Netflix, leading to a new round of questions about whether he plans to continue that series. Probably not, he said. “I gave 16 years of my life to Foyle, three times longer than the war itself,” Horowitz said. “I miss him, too, but I’m not going to say ‘No more Foyle,’ because I learned my lesson with Alex. You never know.”

Never Say Die by Anthony Horowitz. Philomel, $17.99, October 2017, 978-1-5247-3930-0