The Subtle Knife (His Dark Materials, Book 2) - Philip Pullman
(followup to The Golden Compass)
The second book takes place across several worlds which have collided and become mixed. Things get a little more confusing, and it's hard to tell who the "good guys" are supposed to be, and Lyra and a new friend, Will, struggle with what they're supposed to be doing. It seems that there are some characters who are trying to overturn the sense of what is good and what is evil, and it's not clear which side Pullman is rooting for, either. Also, Pullman hints at some more theories about Dust (from Lyra's world), dark matter (from ours), angels, and Specters. Book One, The Golden Compass, could stand by itself: the ending is certainly not final, but a reader could conceivably stop there if they wanted. Book Two, however, raises many more questions than it answers, and I'm eager to find out how this is all going to fit together.