The Subtle Knife
The second book in this trilogy was even better than the first one! The story keeps getting more and more intriguing and the allegory and philosophical discussions are very profound. The addition of the new character of Will and the doorways between worlds added an extra demension (no pun intended) made this book impossible to put down. I immediately reached for the Amber Spyglass (the third book in the series) just as soon as I had finished the last page!