Terry Pratchett 
Only You Can Save Mankind 

IF NOT YOU, WHO ELSE?As the mighty alien fleet from the latest computer game thunders across the screen, Johnny prepares to blow them into the usual million pieces. And they send him a message: We surrender. They’re not supposed to do that! They’re supposed to die. And computer joysticks don’t have 'Don’t Fire’ buttons . . . But it’s only a game, isn’t it. Isn’t it?The first book in the Johnny Maxwell trilogy.