I've always loved fantasy novels, but my favourites are never the kind with dragons and castles. I like the slightly weird fantasy, the kind with interesting concepts and a little bit of sci fi, too. Interworld, by Neil Gaiman and Michael Reaves, is definitely this sort of fantasy.
Joey Harker is an ordinary sixteen year old boy with absolutely no sense of direction. When his tenth grade social studies teacher assigns a test that requires him to find his way to a certain place in town, he doesn't think he has much chance of passing the class--especially not when he walks through a cloud of fog into a world where the McDonald's sign is plaid and his mother doesn't know who he is. Joey soon finds that his Earth is part of the Altiverse, a collection of parallel Earths separated by the In-Between, a strange realm that owes a lot to M.C. Escher, Picasso, and a lava lamp. The Altiverse must be held in balance between those Earths governed by magic, and those governed by science, and it is up to all the Joey Harkers from all the Earths to do this.
I love stories that make use of parallel world theories, and this one is especially clever, with the idea of all the parallel versions of the same person having the power to Walk between worlds. I expect nothing less of Neil Gaiman than thoroughly imaginative storytelling and wonderfully bizarre visuals, and he has certainly delivered on this. It's a great book, meant for young adults but quite universally enjoyable.