James, a former television executive, surpassed regulars on the list, including James Patterson, Danielle Steel and Stephen King, with estimated earnings of $95 million in the year to June 2013, thanks to the titillating trilogy.
"The e-book format was a key factor, giving readers an easy way to purchase sex-filled sequels - and a discreet way to read them in public," according to forbes.com.
Former No. 1 Patterson came in second with earnings of $91 million, while Suzanne Collins, the author of "The Hunger Games" and the five-book series "The Underland Chronicles," was a distant third at $55 million.
The 2012 "The Hunger Games" blockbuster film, an adaptation of the novel, grossed $684 million worldwide.
Fox TV talk show host Bill O'Reilly, the author of "Killing Lincoln" and "Killing Kennedy," came in fourth with $28 million and Steel, with 128 book titles to her credit, was No. 5 with $26 million.
Dan Brown's "Inferno" was the best-selling book of 2013 so far, putting him in ninth place with $22 million. King just made it into the top 10 with earnings of $20 million. The hit U.S. TV series "Under the Dome" is based on King's science fiction novel of the same name.
Forbes based its rankings on estimates on sales data, publishing figures and industry sources.