With more than 1.8 million books in print, Alyson Noël’s the Immortals series has clearly scored a hit with teen readers. In Radiance (Square Fish, Sept.), the author makes her middle-grade debut, launching a paranormal series that’s a spinoff of her bestselling YA series. At the center of the new story arc is Riley, first introduced in Evermore, the first Immortals novel, who is the younger sister of protagonist Ever. St. Martin’s/Griffin will release Dark Flame, the fourth Immortals novel, next month.

The idea for the middle-grade series came from Square Fish publisher Jean Feiwel, says Noël. “When Jean proposed this to me, I jumped at the chance to write about Riley again,” she recalls. “Riley ended up having a bigger part in Evermore than I’d envisioned because I enjoyed writing her so much.”

But the author had an initial moment of self-doubt. “Before I began to write Radiance, I went into a panic,” she says. “I’m so used to writing for teens, and that’s my state of mind anyway. No matter how old I really am, I am a teen in my head, so I thought, ‘How am I going to write for 12-year-olds or in the voice of a 12-year-old?’ But I told myself not to worry about Riley’s age, just recreate the character, and that worked.” Noël calls Radiance “one of the easiest books I’ve ever written. It really flowed, and I enjoyed the entire process. And, honestly, that is not true of every book I write.”

Noël published her first YA novel, Faking 19, back in 2005, while working as a flight attendant. “We’d have a lot of downtime between flights, and of course delays, so I was constantly writing—on cocktail napkins or anything I could find,” says the author, who had been determined to become a writer since reading Judy Blume in sixth grade. She turned to writing full-time after September 11: “I knew that my job as flight attendant would never be the same. When we had to take pay cuts and do bomb searches, I realized that it was time to get serious about my writing.”

After writing six subsequent novels, Noël decided to venture into paranormal fiction with the Immortals and didn’t know how her readers would respond. “I didn’t know what to expect, but it was a great and almost surreal moment when I realized that readers embraced it.”