Mystery author Caroline Cooney doesn’t keep secret the writing approach that helped her sell 10 million books.

“I write high-velocity and high-suspense stories with pretty happy endings,” said Cooney, a writer who regularly proves popular with the difficult-to-reach, 10- to 14-year-old age group.

“You can add romance, which is always pleasing to the girls but not the boys,” she said.

Tuesday morning, Cooney will be in Lawrence to share literary insights with 100 teens at Southwest Junior High School. She’s on a tour promoting her book, “What Janie Found,” the fourth and final installment in a series about a kidnapped child.

“People always love to hear where writers come up with ideas,” she said. “And, I want to encourage them to write.”

Her appearance is sponsored by The Children’s Book Shop, 937 Mass. Sue Vance, store manager, said signed copies of Cooney’s books would be available at the store.

Cooney, 54, said “68 or 69” of her books had been published. She took up writing nearly three decades ago to occupy free time while caring for her young children.

“Sitting home with babies,” she said, “I had to find a way to entertain myself.”

Her first published piece was in Seventeen magazine, and her first book, “Safe as a Grave,” came out in 1979. She concentrated on romance novels and horror stories before turning to the young adult audience.

She’s never understood why some writers find the work difficult, she said. It’s normal for Cooney to average two or three books a year.

“I love writing and do not know why it is considered such a difficult, agonizing profession,” she said. “I love all of it, thinking up the plots, getting to know the kids in the story, their parents, backyards, pizza toppings.”

The tale of Janie Johnson, who was kidnapped as a child, was intended to be a stand-alone book, “The Face on the Milk Carton.” Instead of whipping out a sequel, she sponsored her son-in-law’s local stock car, which had the book cover painted on the vehicle.

That satisfied Cooney, but it didn’t appease her youthful fans.

Hundreds of children sent letters to Cooney begging the Westbrook, Conn., author to resolve lingering questions about Janie’s life. Readers worried about the character as they would a real person.

Eventually, Cooney relented. She wrote three sequels.

She said the quartet of Janie novels interested teen-agers because each touched on questions young readers asked themselves: Who am I? What am I doing here? Will I make good choices in adverse times?