Armada - Ernest Cline

It's just another day of high school for Zack Lightman. He's daydreaming through another boring maths class, with just one more month to go until graduation and freedom - if he can make it that long without getting suspended again. Then he glances out his classroom window and spots the flying saucer.

At first, Zack thinks he's going crazy. A minute later, he's sure of it. Because the UFO he's staring at is straight out of the videogame he plays every night, a hugely popular online flight simulator called Armada - in which gamers just happen to be protecting the Earth from alien invaders. But what Zack's seeing is all too real. And his skills - as well as those of millions of gamers across the world - are going to be needed to save the Earth from what's about to befall it.