"The Subtle Knife" is the second book of the "His Dark Materials" series. The story starts here on Earth with a 12-year-old boy Will. Unknowingly, Will murders a strange man who used to come regularly to search his house and now he must escape. Soon he discovers a window which takes him to another world.
Here he meets another girl called Lyra who has come from another world just like Will. This strange world has some surprises for them. The world is stalked by soul-sucking invisible fiends called Spectres, who attack only adults and children can't see them.
As the story unfolds, the readers come to know about the prophecy of the witches involving Lyra. Will sets forth to search for a special knife. Mrs. Coulter tries to find her daughter. Lee Scoresby sets off to find Mr. Grumman. The witches discover that Lord Asriel is building his army against the Authority... and so the story goes on unfolding surprising the reader every moment.
It is worth a read, so go ahead and try out!