The Subtle Knife (His Dark Materials #2) by Philip Pullman.

"Lyra's adventures continue as she follows her Father across the bridge in the sky to a different world, where she meets Will. A tenacious, determined young man, about her own age, but whose from a different world. It's a similar world like hers, but exactly the same. He's on a mission to find his Father, who disappeared 12 years ago on an expedition in the North. And of course, though they try their best to work together, there are forces at play in this brewing war that Lyra and Will can't even comprehend, let alone fight against. It's truly a cliffhanger and I'm interested to see how it all ends."

3 Stars

Pros: More or less the same cast of character so not too many new people to remember/Will is passionate, straightforward and caring, in his own gruff way and he helps Lyra learn how to see the world from others view, and not just through her own self centeredness. They make each other better. 

Cons: It all seems very convenient that people like Lord Boreal can travel between worlds ever so easily while others, like Will need a special, magical knife/When Will and his Father finally meet...yeah...don't even get me started. 

Full Review: 

Lord Asriels plan worked, or at least, as far as Lyra can tell, it did. But she's left reeling after the events of The Golden Compass and she follows her father over the bridge in the sky, intent on destroying Dust, since that seems to be what has all the adults in an uproar. 

Meanwhile, in our world, Will Parry, a 13 year old boy, leaves his ill mother in the caring hands of his former teacher and then, he goes back home. Numerous times now, men have come to the house to pester his mother and search the place and Will knows what they're looking for. They're letters from his Father, who was a famous explorer, who disappeared 12 years ago. Will knows that something important is in those letters and he has to keep them away from the men who keep searching for them. With his mother safe, he sets off, intent to run away, not sure of his destination but just knowing he has to disappear somewhere. 

And disappear he does, quite literally. He manages to find a "window" into another world. It's completely empty and he's sure that he's found his safe place. He's content to stay there for as long as it takes, that is, until he runs into Lyra, who came to the same world after following her Father. So apparently, Asriel opened up a window to another world, or multiple worlds, and this empty world that Lyra and Will are in, seems to be some sort of stopping point. But that's also not true, as it turns out there are children in this world, who make themselves known eventually to Lyra and Will. 

Of course, Lyra and Will are curious as to where all the adults are. And they eventually find out. Citagazze (this world of children), is not so much the paradise they thought. It's filled to the brim with Spectres, creatures who feast on something in adults, whether it be their essence, their soul, what have you. But whatever it is that the Spectres eat, the adults are left empty shells, indifferent, like zombies. The Spectres don't bother the children, but once you reach the age of puberty, they'll find you. So much in the same way that in Lyra's worlds, your Daemon takes a permanent shape when you reach a certain age, so too do the Spectres hunt you in Citagazze. 

And during all of this, Lyra continues to consult the Alethiometer to try and figure out what's going on. It tells her to trust the scholar, to help Will find his Father and she does all these things. But nothing's ever easy or goes exactly according to plan. 

The scholar she meets is a young woman named Mary Malone, who's studying dark matter in a small department at a museum that Lyra visits while she's in Will's world. Lyra tells her all about the Alethiometer, the Dust and it turns out that Dust is in fact, these Shadow particles that Dr. Malone has been studying. It's all one in the same and after Lyra and Will disappear back to Citagazze, she takes Lyra's advice and manipulates the instruments in her study so that she can "talk" to them. And she discovers that these shadow particles are sentient, they understand and can communicate with her. And it turns out that they are Angels, Hell bent on vengeance for the disastrous war between Lucifer and God and the story of the Fallen Angels. 

Armed with that knowledge, Mary Malone sets off to find Citagazze and "act the serpent" as the Angels tell her. And meanwhile, guarded by Serafina Pekkala and a number of other witches, Lyra and Will set off through Citagazze to try and find Will's Father. 

Again, the ending of The Subtle Knife is as much of a cliffhanger as The Golden Compass so even though it's getting a little cumbersome with the whole fallen Angels, Spectres and other such numerous plot lines and such in play, you just have to keep reading to find out what the Hell is actually happening. Suffice it to say, I'm interested to see how and if The Amber Spyglass wraps everything up in a neat, tidy bow. We shall see. We shall see.