The Homeward Bounders by Diana Wynne Jones

The plot of the book is Jamie sees something he shouldn't - what appears to be two hooded figures standing over a map of the world in a strange room. As punishment, he is sent into other worlds with the promise that one day he may be allowed 'back into play' if he finds his way home. Until then no one can interfere with his journey, but there's nowhere for him that he can belong. For every time one of the mysterious Them make a move, Jamie is forced into a new world.