Pre-Owned The Blob That Ate Everyone (Goosebumps, No 55) - R.L. Stine

Zackie Beauchamp wants to be a famous horror story writer. He's writing a story about a giant blob monster. A pink slimy creature who eats up an entire town. Then Zackie finds the typewriter in a burned-down antique store. He takes it home and starts typing but there's something really odd about the typewriter. Something really dangerous because now every scary word Zackie writes is starting to come true