Ghosthunters and the Incredibly Revolting Ghost! by Cornelia Funke
Watch out for ghost goo! Tom's stuck at home with his horrible sister, Lola. But there's something stickier hiding out in his cellar - a slime-dripping ghost called Hugo! But harmless Hugo is in the goo too, because he's being haunted - by a much bigger, badder, Incredibly Revolting Ghost. Lucky for Tom that his Granny's best friend just happens to be a world-famous ghosthunter ...