The Subtle Knife is volume two of a trilogy called His Dark Materials. Volume one was called either The Golden Compass or The Northern Lights, depending on whether you picked up a British or American edition. Either title fits, because both things have a lot to do with the development of the plot. In the first book we meet a girl named Lyra, who lives in a world where beings have souls (or daemons) which live externally with them, shapeshifting entities which assume a permanent form only when it becomes clear what sort of adult a person is going to become.
Lyra becomes involved in the plots of her estranged parents. Her mother is a Power in the Church which dominates her world and is connected with a nasty experiment in which children are separated from their daemons in order to gain access to a type of energy. Her father opposes the Church, but is working on his own plots, having to do with what we would call the Northern Lights, but which is called Dark Matter in her world.
When we left her, Lyra had walked off into the Aurora and was crossing between worlds.
The Subtle Knife does not immediately pick up that plot thread, but instead introduces us to our own world and to a boy named Will, who is looking after his brain damaged mother and trying to find out something about his missing father. Other people are trying to find out about his father as well and 12 year old Will is placed in the incredible position of having killed one of them, quite by accident, before we are very far into the story.
On the run, he stumbles across something which which could only be likened to a tear in the air, on the other side of which he finds himself in the same world to which Lyra and her daemon, Pantalaimon, had journeyed. In this world malevolent beings called the Specters are separating adult humans from their souls, giving them a spiritual and intellectual lobotomy. Children are safe until they hit a certain stage of puberty, but that means that the only functioning humans in the area are children, and the end result is a most unstable society.
Meanwhile, back in Lyra's world, which has the feel of late 19th century England about it, those who were Lyra's allies in her original quest are trying to find out what happened to her, and those who were trying to capture her are trying to do the same. We begin to get the impression that there might be a link to the three worlds that we have seen so far, that there might be more, and that all of them might be in danger because of events in Lyra's world, or maybe events in that in-between place where Lyra and Will are trapped.
Certainly Lyra's mother, Mrs. Coulter, wants the artifacts that Lyra and Will have found. The first is the Golden Compass, or altheiometer, with which a talented or learned individual may discern truth, see the unseen, and learn all manner of things in response to the right questions. Lyra has that, and she seems to be its natural guardian. After their initial adventures in the Children's World, Will becomes the Bearer of the Subtle Knife, a mystical device with two edges. One will cut effortlessly through any material. The other will cut and seal the ways between the worlds.
Evil forces are after these devices. What they intend to do with them is anyone's guess, but the horrors of the Children's World are a clear indication of what they can do in the wrong hands, for it was a wielder of the Knife who left the Specters in.
There's much more in this book. There are adventures with witches and last ditch battles on mountain tops, as well as aerial chases and devious intrigues. Our two young protagonists have a lot to learn about each other, and the mystery of Will's father's life and disappearance in Alaska (yes - a northern connection) could be worth a story all by itself.
Pullman leaves us with a a lot of unresolved questions and what amounts to another cliffhanger at the end of this book. The good news is that the final volume, The Amber Spyglass, is already available.
Individually and together, the books have been winning a lot of awards and attracting a lot of attention. They are often compared to J.K. Rowling's Potter books in terms of their impact on young readers, though they are quite different in theme, content and style. Pullman's novels have a touch of Mervyn Peake in them, where Rowling's feel a bit more like Roald Dahl meeting Enid Blyton. His Dark Materials also takes place in a much shorter time span, so there isn't same degree of change in the characters. Nor are they the complete focus of the stories. Unlike Harry Potter, the action does not revolve solely around Lyra and Will; there is a lot going on in other places.
Still, I'd wager that if you like Rowling (whether adult or youngster) you'll also like Pullman.