James talked about the surprise hit, which started out as an e-book, in her first prime-time television interview with "20/20" co-anchor Elizabeth Vargas.
"It's a contemporary, romantic fiction, with quite a lot of sex in it," James said. "I think it's the love story [that appeals to people. ... I hope it's because of the love story."
The book and its two sequels -- "Fifty Shades Darker" and "Fifty Shades of Freedom" -- have been dubbed by the media "mommy porn."
The series follows dashing billionaire Christian Grey as he uses bondage, discipline, sadism and masochism in his relationship with his young, inexperienced lover Ana.
ABC News said James, a former television executive who writes under a pseudonym, began posting her writing on a fan fiction Web site for Stephenie Meyer's "Twilight" books.
She called her "Fifty" series of novels her "midlife crisis."
"It's all my fantasies in, in one thing," she said. "I was inspired by Stephenie Meyer. She just flipped this switch."
Like Meyer, James has signed a deal to have her books brought to the big screen, ABC News said.