Let me just start things off by saying that this cover kind of terrifies me a little. I know that monkey-dogface creature in the background is likely meant to be Mrs Coulter's daemon, but it looks more like the stuff of nightmares. So it's a good thing I finished The Subtle Knife by Philip Pullman in one night, straight. This is the second book in the His Dark Materials trilogy.
We start off first thing with a new character, twelve-year-old Will Parry, who lives in the England of our world. Some men are after something in his house, and it's tormenting his mother, so he stashes her with his old piano teacher and takes off with the thing he suspects the men are after. He makes it to Oxford and finds what looks like a window, but in the middle of the air. Not a regular window, though...you can only see into it from a certain angle. Well, he's basically on the run and trying to disappear for a while, so obviously he goes on through this thing and finds himself in another world entirely.
Now I know what you're maybe thinking, but this isn't Lyra's world from The Golden Compass, either. If you recall from the end of that book, Lord Asriel was looking into the matter of Dust, and all kinds of crazy shit happened, and Lyra decided to try and follow him through the portal or whatever it was that he'd managed to open. Anyway, so this place Will stumbled into isn't Lyra's world because she's found her way into it, too. This is where they meet each other and sort of team up.
It's almost serendipity that the two have found each other, because she wandered into this world trying to follow her father, and Will is hoping to find out about his own father, who went missing when he was just a baby. John Parry was an ex-Marine-turned-professional-explorer, and it's possible there is a connection between his father and the men who have been harassing his mother.
So Will and Lyra kind of go back and forth an awful lot between Will's Oxford and the almost-deserted city in the world they came into. Lyra of course gets them mixed up in all kinds of trouble when a man notices her with the alethiometer in a museum and she meets with a physicist to discuss Dust. All the while, though, there are great changes happening in all worlds, and Will is about to find out that it may be more than mere chance that brought him through that astral window and into the path of Lyra and Pan. He may have a much bigger role to play in what's about to come than he would ever have imagined.

