In The Secret History of Twin Peaks (Flatiron, Oct.), Mark Frost fills in some of the backstory of the cult TV series before its revival on Showtime.

Were you surprised by the persistent intensity of the show’s fans over the past two-plus decades?

I’ve always been astonished by the level of loyalty and obsession that people have brought to the show, going all the way back. I recall someone bringing me 600 pages of printouts from an early AOL forum back then—of people spinning theories and discussing solutions, and I realized that we had come of age at exactly the same time as a different way of expressing fan interest. On my book tour, I’ve met a wide range of fans—those who watched it when it first aired, some who were brought up on it by their parents, and millenials encountering it via streaming or the DVDs, and that’s a coalition I think we can maintain with the new episodes.

Did the idea for the book predate the deal to produce a third season of Twin Peaks?

It did. I actually wanted to write a book like this 25 years ago—the notion of a book, which went backwards in time and which set the context for the world we had created, was really appealing to me. But I didn’t have time to do it then, because we were busy making the show, and when it was cancelled, there didn’t seem to be much reason to do it. More recently, I didn’t actively turn my mind to it until after we’d finished writing Season Three. I’ve had a long relationship with Bob Miller, who founded Flatiron Books. He’d been my publisher at Hyperion going all the way back to 2000, and Bob made a great preemptive offer on the book.

Why did you decide to write the novel as a collection of letters, narratives, and documents?

That form of storytelling seemed to be the most advantageous to what I wanted to pull off—to create a mosaic of voices rather than just have an omniscient narrator. I also wanted to build into the book the same kind of sense of an unfolding mystery that the show was known for.

Did the concept of using the Lewis and Clark expedition as part of the past history of Twin Peaks go back 25 years, or was that a more recent addition?

For the most part, that’s a more recent addition. Lewis and Clark were the first contact that the indigenous culture of the region where the series is set had with the U.S.’s Western expansion, and so it made sense to me to include them.