I was a little skeptical when I learned that Pullman would be writing a second trilogy, the Book of Dust, since His Dark Materials ended satisfyingly. But the world of his books is a vast one, and it has been a delight to revisit it and discover how much more he has to say: about human consciousness and authoritarianism, the value of romanticism, the dangers of a changing climate and the tragedy of the ongoing European refugee crisis. “La Belle Sauvage” (2017) serves as a prequel of sorts to the original trilogy. “The Secret Commonwealth” (2019) takes place when Lyra is 20 and the events of the first trilogy have long passed. The third and final book, “The Rose Field,” comes out Oct. 23 and concludes Lyra’s story. Here’s what you need to know before you pick it up.