Invasion of the Body Squeezers: Part 1 (Goosebumps Classics #41)

"Jack Archer is convinced something strange is going on at his neighbor's house. Mr. Fleshman always wears all black, hardly ever leaves his house, and never says hello to anyone. Then Jack spots a mysterious creature through Mr. Fleshman's window. It's big and gray, with rotten-looking flesh. Jack has never seen anything like it on Earth. Could it be -- an alien? If it is, Jack will have to convince everyone that an invasion is underway!"--