His Dark Materials Trilogy - Book 2 - The Subtle Knife
Pullman, Philip
At the end of The Northern Lights, Lyra Silvertongue watched in fear and fascination as her father, Lord Asriel, created a bridge between worlds. Lyra and her daemon, Pantalaimon, are now lost in an alternate universe where they meet Will Parry, a fugitive from a third universe. Will has found a small window between Cittagazze (a place where children roam unchecked but invisible Specters suck the spirit out of adults) and his Oxford, which, with its Burger Kings and cars, is frighteningly different from the Oxford Lyra knows.