My Best Friend Is Invisible is the 57th book in the original Goosebumps series.
The cover illustration depicts an invisible Brent Green having pizza and a soda as Brutus, Sammy's cat, is standing up and screaming in a comical way. Tim Jacobus' signature, which is very difficult to see from afar, is on the bottom part of the pizza box that is facing Brutus. If one looks closely at Brutus' legs, you'll see that they are slightly transparent for reasons unknown.
Synopsis[]
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!