The Homeward Bounders — Diana Wynne Jones

If he finds the right world, Jamie can get Home again.

When Jamie stumbled upon the powerful Them playing Their mysterious games, They threw him out to the Boundaries of the worlds. Since then, he's been yanked from world to world, doomed to wonder in hope of one day finding his way back to his own city.

Bit by bit, though, Jamie realizes there are rules They have to play by. He forms an alliance with two other lost Homeward Bounders -- bitter, powerful Helen and demon-hunter Joris -- and takes a desperate chance, hoping that the three wanders can find a way back to their home worlds at last.

Once he becomes a pawn in a game played by a powerful group he calls Them, 12-year-old Jamie is repeatedly catapulted through space and time.