Ferrante closely vetted and approved the eight-part Italian television adaptation of the first book, My Brilliant Friend, they said.
The first episodes will be shown in the United States on HBO in November, with 24 further episodes planned for the rest of the saga.
The four novels following two friends Elena and Lila from their dirt-poor childhoods in postwar Naples have become an international publishing sensation, with millions of copies sold.
Screenwriter Francesco Piccolo said Ferrante -- who has tried to keep her real identity secret -- was "very close to the project from the beginning", even choosing director Saverio Costanzo.
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"We can't tell you who Elena Ferrante is because we don't know ourselves despite the fact that we worked with her.
"She followed the project very closely from the beginning, like a kind of supervisor," Piccolo added. "You can see in the emails she had with Saverio that her confidence in him grew as it went on," he told reporters after the first two episodes were show at the festival.
Costanzo said that although he used the first book "as a compass, I had to change the order of events, and lump things together. Elena uses a pen to order things and I had to use a camera."
But he said he hoped "readers take up the film at the point where they put down the book. If that happens the credit will not be ours, but the brilliance of the books.
"They were so coherent that adapting them was easy. It was like slipping into old shoes that they keep you grounded."