From the Times:
“This is a big subject and I’m writing a big, big book in order to deal precisely with that question,” he tells the magazine. “I don’t want to anticipate it too much by switching a light on the answer now. The interesting – the curious – question is, if people can be helped by something that is palpably not true, is this better than denying the thing that is not true and not being helped?”
According to the article, The Book of Dust isn't going to be a reference book, but a novel. No word as to the pub date, though.