"Goosebumps Horrorland #4: The Scream of the Haunted Mask" by R. L. Stine

What should Carly Beth be for Halloween this year? TERRIFIED! Late at night, an ugly green mask is mysteriously calling out to her, and ugly green masks don’t like to be ignored. If Carly Beth survives the night, even a scary theme park might sound like a vacation. Or maybe not! At HorrorLand, every night is Halloween. And those monster masks? They aren’t masks.