HisDarkMaterials.org

The Book of Dust is intended to be a companion to the His Dark Materials trilogy. It will explain Lyra's world and, most importantly, the background histories of secondary characters like Lee Scoresby and Serafina Pekkala.

Rumors about The Book of Dust practically date back to the publication of The Amber Spyglass in 2000, but Philip Pullman has set no possible publication dates and has worked on several separate projects in the interim. The Book of Dust is definitely in the process of being written - the press release of Lyra's Oxford mentions it by name - but there's no way to gauge how near it is to completion or when it will finally be published.

"The Book of Dust will not be a simple reference book--far from it. I want to go into the background of Lyra's world, and the creation myth that underpins the whole trilogy, and to say something about some of the other characters, and about the alethiometer and the history of the subtle knife, and so on. Furthermore I want it to be richly illustrated. It'll be story-driven, not reference-driven, and I'll need to brood over it in silence before I find the right form for it."

- Philip Pullman, "Letter from London", Publisher's Weekly, March 05 2001