I Live in Your Basement! is a novella written by R. L. Stine. It was first published in 1997 and was the 61st book in the original Goosebumps book series.
Description[]
Sick of his overprotective mother keeping him from having fun, Marco sneaks off to play baseball — and ends up getting hit in the head. While recuperating at home, Marco begins getting strange calls from someone called "Keith" who wants someone to take care of him, and the more Keith is around Marco, the more Marco's reality becomes warped.