A dozen years ago, the acclaimed British author Philip Pullman teased his legions of fans with a cryptic message posted to his website: “The Book of Dust,” he wrote, “is under way.”

Finally, ‘Dust’ is just months away. Pullman’s next book will be published simultaneously in the U.S. and the U.K. on October 19. There are still many mysteries to be revealed, but here is what his publishers shared about the new series (yes, series) today:

The Book of Dust is “a work in three parts.” The titles for each volume, if they have been decided upon, are closely guarded secrets. (For a book 12 years in the making, it is probably safe to assume there will be not only a “cover reveal,” but a “title reveal,” too.)

Knopf Books for Young Readers, the U.S. publisher, plans a first printing of 500,000 copies. Penguin Random House Children’s and David Fickling Books will publish the U.K. edition; Listening Library will simultaneously publish an audiobook edition.

Plot details are under wraps until publication, of course, but in a statement provided by his publishers, Pullman shared that the book is set 10 years before the events of The Golden Compass and centers on Lyra Belacqua, heroine of the His Dark Materials trilogy, a series that has sold more than 17.5 million copies in 40 languages since its debut in 1996.

Pullman has also coined a new publishing term to describe the forthcoming work. It is neither a prequel nor a sequel nor a standalone. “In fact, The Book of Dust is.... an equel,” Pullman wrote. “It doesn’t stand before or after His Dark Materials, but beside it. It’s a different story, but there are settings that readers of His Dark Materials will recognize, and characters they’ve met before. Also, of course, there are some characters who are new to us, including an ordinary boy (a boy we have seen in an earlier part of Lyra’s story, if we were paying attention) who, with Lyra, is caught up in a terrifying adventure that takes him into a new world.”

Published between 1996 and 2000, the His Dark Materials trilogy is widely regarded as a modern classic. It has been adapted for television and for film, with Nicole Kidman starring as Lyra’s villainous mother, Mrs. Coulter. Individually, the three books won several literary prizes, among them the Carnegie Medal, the Guardian Children’s Book Award, and the Whitbread (now Costa) Book of the Year Award—the first time that prize was given to a children’s book.

“The announcement of Philip Pullman’s The Book of Dust has been a highly anticipated event for our industry,” said Barbara Marcus, president and publisher of Random House Children’s Books. “Pullman is an extraordinary writer whose thought-provoking works deserve all the acclaim that has been bestowed upon them.”

Before the launch of the new series, Random House will repackage the His Dark Materials books in paperback and issue two companion books—Lyra’s Oxford and Once Upon a Time in the North—for the first time in softcover. There are also plans for a second graphic novel based on The Golden Compass.

Pullman has been at work on the new series since at least 2003 (the last book in the His Dark Materials trilogy, The Amber Spyglass, was published in 2000). He has talked about it—and posted tidbits about its plot—for more than a decade, although the scope of the work seems to have changed substantially since he began.

“I’ve always wanted to tell the story of how Lyra came to be living at Jordan College, and in thinking about it I discovered a long story that began when she was a baby and will end when she’s grown up,” Pullman wrote. “This volume and the next will cover two parts of Lyra’s life: starting at the beginning of her story and returning to her 20 years later. As for the third and final part, my lips are sealed.”

His motivation to return to the story? In a word: Dust. Pullman’s Dust is a (fictional) particle, invisible to the human eye (unless you have an amber spyglass.) It is the source of great anxiety in Lyra’s world because the Authority (the church) believes it is physical evidence of original sin. Lyra (and, not coincidentally, Pullman) believe differently.

“Questions about that mysterious and troubling substance were already causing strife 10 years before His Dark Materials, and at the center of The Book of Dust is the struggle between a despotic and totalitarian organization that wants to stifle speculation and inquiry, and those who believe thought and speech should be free,” Pullman wrote. “Little by little through that story the idea of what Dust was became clearer and clearer, but I always wanted to return to it and discover more.”