Ghosthunters and the Muddy Monster of Doom! (Ghosthunters, #4)

More frightening fun for 7-to-10-year-olds from the phenomenal Cornelia Funke! Book 4 in the spooky-silly GHOSTHUNTERS series.
Intrepid ghosthunter-in-training Tom has battled Incredibly Revolting Ghosts, Gruesome Invincible Lightning Ghosts, and a Totally Moldy Baroness. But it ends up nothing--NOTHING--is more frightening than...final exams. To earn his Ghosthunting Diploma, Tom's got to nab a Deadly Messenger: the hardest ghost of all to catch. And while on the hunt, he stumbles into the yucky, mucked-up lair of a mud-dripping minotaur demon who's assembled a ghoul army with the goal of world domination. Suddenly there's a whole lot more at stake than Tom's graduation: This is one test the team can't fail!