My Best Friend is Invisible (Goosebumps #57) by R.L. Stine - Vintage Paperback

HE'S OUTTA SIGHT...FOR REAL!

Sammy Jacobs is into ghosts and science fiction. Not exactly the smartest hobby—at least not if you ask Sammy's parents. They're research scientists and they only believe in real science. But now Sammy's met someone who's totally unreal. He's hanging out in Sammy's room. And eating his cereal at breakfast. Sammy's got to find a way to get rid of his new "friend." Only problem is...Sammy's new "friend" is invisible!