This is R.L. Stine's book, Attack of the Mutant from the Goosebumps series. Book is in never-read condition, although it does have a slight crease down the side.
Skipper Matthews has an awesome comic book collection. His favorite one is called The Masked Mutant. It's about an evil supervillian who's out to rule the universe!
Skipper can't get enough of The Mutant. Until one day he gets lost in a strange part of town. And finds a building that looks exactly like The Mutant's secret headquaters. A building that appears and disappears.
Has Skipper read one too many comic books? Or does The Masked Mutant really live in Riverview Falls?