Ms. Pascal is not unsympathetic. “Sweet Valley was their adolescence and I’ve done some very radical things,” she said. “But you are different from that inchoate person of 16, and you have to allow that change to those characters.”

Much about the original books has remained the same. The series, which was introduced in 1983, chronicled the high school dramas of the Wakefield twins, Jessica, the flirtatious diva, and Elizabeth, the virtuous goody-goody. (Both were popular and beautiful.)

In “Sweet Valley Confidential,” which was released March 29, the idyllic Southern California town of Sweet Valley is still the home of many of the original cast of characters from the series, including the rich and snobby Lila Fowler, the gossipy Caroline Pearce, the nerdy Enid Rollins and the mysteriously reformed villain Bruce Patman.

Ms. Pascal, who is energetic and platinum-haired at 72, dreamed up the series in the early 1980s, even though she had never visited California. She grew up in Queens, studied journalism at New York University and wrote for magazines and soap operas before starting a career as a novelist writing for the young-adult market.

Sitting on a velvety powder-blue chair in the living room of her apartment in Midtown Manhattan, a warmly lighted space decorated in feminine tones of lemon and gold, Ms. Pascal recalled how she easily sold the first six books of the series after brainstorming with her agent, Amy Berkower.

“There was nothing like this at the time,” Ms. Pascal said. “There were romance books, but this was different. And one of the great differences was that these books were girl-driven, and this was very important to me. So with two girls I had girl power. That gave it great appeal.”