His Dark Materials

A trilogy of books written by Philip Pullman.

The first book, 'Northern Lights' is set in a universe parallel to and similar to our own. The first and most immediate difference, and the difference adored most by self-indulgent Fangirls, is that everyone has a daemon, which is akin to a sort of visible extension of the soul, and takes animal form (although the daemon of one Master of Jordan College is recorded as having been in the form of a human woman). When you are a child, your daemon can change form at will, but once you hit puberty, it settles, and chooses one form. Said form will typically say something important about your nature - servants often have dog daemons. Other differences between that world and this are that the Church has greater power, and that there are talking bears, the Panserbjorne that live in the Arctic. Technology in this world is rather steampunkish.

The main character of this book is Lyra Belacqua, a young girl, intelligent but somewhat given to lying, who is given an important instrument - the aleithiometer - by the Master of Jordan College, where she has lived most of her life, and sent to live with the beautiful and frightening Mrs Coulter.

The second book, 'The Subtle Knife', introduces us to Will Parry, a boy who lives in our world. He discovers a portal to Cittagazze, a city in another world, and meets Lyra, who has travelled there following the events of Northern Lights.

The third book, 'The Amber Spyglass', is the book in which, essentially, we find out what's really happening. Lyra and Will are separated - Lyra with Mrs Coulter, who is drugging her to keep her from the Magisterium (the Church, essentially) and Will in Cittagazze, talking with two angels.

The rest of the plot is rather too complicated and spoilerriffic to go into here - suffice it to say that the trilogy is excellent.

See also The Movies - Northern Lights was made into a film named The Golden Compass.