Stan and the Toilet Monster — Steve Shreve

It came from the sewers....
It's just another day on Calamity Avenue. Stan Hankie and his friends are playing baseball, when the ball - their last one - falls into the sewer drain. Everyone knows that there's a monster in the sewers. Stan and his best friend, Larry, go down in search of the ball, anyway. But instead of a baseball, they find something else. Then strange things start happening around town. Is a creature from the sewers coming after them? Can Stan and Larry figure out what's going on before the whole town is destroyed.