I Live in Your Basement! is the 61st book in the original Goosebumps series.
The cover illustration shows Keith in his monster form in the basement.
Synopsis[]
"Don't do this! Watch out for that!" Marco's mom thinks the whole world is a danger zone. She won't even let Marco play softball. But Marco wants to have fun. So he sneaks off to a game. And that's when it happens. He gets hit in the head with a baseball bat. Now things are getting really fuzzy. Really scary. Because when Marco gets home, he gets the strangest call. From someone who says he lives in Marco's basement...