The Golden Compass closely followed Lyra Belacqua. I don't think there were any chapters in which she did not appear, if there were, they were few and far between. The Subtle Knife, the second book in the trilogy, does not follow suit. It opens with a fairly lengthy chapter about a young boy named Will Parry, whose mother has some serious cognitive disabilities that require him to look after her. His father, although alive, is not in the picture. It is not clear what happened to him. Will gets an elderly neighbor to watch his mom for a few days, because he "has something to do". Upon returning to his empty house, he is attacked by to men, one of whom he unwittingly kills. As he is on the run, he notices a cat acting weird and then leaping forward and simply disappearing. He approached the area where the cat was, and realizes something is not right. He steps forward, through a "window" into another world.
At the end of The Golden Compass, Lord Asriel -- whom Lyra discovers is her father -- has opened up a "bridge" between worlds. The idea is that there are many worlds that inhabit the same space -- Earth. While there are similarities among these worlds, they have major differences as well.
Will and Lyra meet in a world that is strange to the both of them, in a town called Cittagaze, a town that appears to be inhabited solely by children. It turns out that the adults have fled the town because of specters that suck the souls out of adults, but pass right over children. Lyra starts to notice some connectivity between her world, this one, and Will's. Her daemon, Pantalaimon, is comparable to what Will would describe as his soul or conscience. The elusive dust, which played such a big part in the first book, closely resembles original sin. Lyra comes to the realization that she must help Will find his father, an arctic explorer who inexplicably disappeared with his group a few years back.
The subtle knife turns out to be a implement that allows one to cut windows into other worlds. However, it chooses its owner in a rather gruesome way. The knife chooses Will, and along with Lyra they set out to find his father. My question is, if there are countless "worlds" on Earth, what controls what world one cuts into? This is never really explained, as Will and Lyra seem to stay within three or four worlds throughout the book.
Compared to the first book, this one is complex. There are many more characters, and the plot is much more complicated. Everyone always says that the Harry Potter series gets more grown-up with each book. This may be true, but the His Dark Materials trilogy easily trumps the Rowling series in this aspect. Will spends the better part of the book bleeding profusely. People die left and right, including principle characters. There is a noticeable degree of sexual tension between Lyra and Will. And it becomes apparent about halfway through the book that many of the main characters are attempting to wage a war against God himself -- referred to as The Authority. It is not clear which side of this battle Will and Lyra will fall on, but I suspect that they will join those fighting The Authority. I find this concept interesting, and rather crazy for a children's novel. I have heard that the next book, The Amber Spyglass is even darker, weirder, and more complex. I am really curious whether Pullman can finished this series in an effective and satisfying way.