His Dark Materials

Although ostensibly for children, the appeal of the novels is equally compelling for adults. Pullman's universe, like that of many other contemporary fantasy writers such as Michael Moorcock and Clive Barker, is multilayered and multifaceted, with possibilities for characters to slip between them.

Warning: Wikipedia contains spoilers.

The novels draw heavily on gnostic ideas. The three major literary influences acknowledged by Pullman himself are the essay On the Marionette Theatre by Heinrich von Kleist; John Milton's Paradise Lost and the works of William Blake.

In Northern Lights (published in the USA as The Golden Compass), the young girl, Lyra Belacqua, journeys to the icy wastelands of the far North to save her best friend Roger, and other kidnapped children from experimentation by evil scientists and a revisionist church in an alternate world to our own.

In The Subtle Knife, Lyra journeys to the otherworld called Cittágazze, where she meets Will Parry. Together they travel from world to world and discover the Subtle Knife of the novel's title, and begin to uncover the truth of their own destiny.

In The Amber Spyglass, the series concludes with Will and Lyra visiting the Land of the Dead and releasing the dead souls from their captivity, the destruction of the Subtle Knife, and the sealing of the passageways between the worlds by the angels.