Shadow People : What are the strange beings that Alex can see? (Fiction Express) -- Paperback / softback

Alex is an ordinary boy who keeps seeing weird things. They look like people, only they're not really there. A night-time knock at the door reveals the shocking truth behind Alex's visions. He is needed- to help stop an invasion! Alex joins a task force of young people battling to save the planet! But just what is it they are fighting against?