Everlost - Neal Shusterman

...but their souls don't exactly get where they're supposed to get either. Instead, they're caught halfway between life and death, in a sort of limbo known as Everlost: a shadow of the living world, filled with all the things and places that no onger exist. It's a magical, yet dangerous place where bands of lost children run wild and anyone who stands in the same place too long sinks to the center of the Earth.