Ghoulhunting is a riot, but exams are not and Tom is taking his third ghosthunting diploma. When he is sent to Bogpool, a misty, muddy village, it seems it's bog-standard ghostcatching and his diploma is in the bag. But he has entered a dirty trap, and as the village begins sinking into the mud, a beastly ghost of death arises!

Ghost Hunters and the Mud-Dripping Monster! by Cornelia Funke (soft, good)