The Boy Who Became a Monster — Jonathan J. Drake

When Harry is transformed into a Gluey Glubber monster, it's left to his friends to try to restore him to normal. To do this they need to discover the secret behind Creepy Keith's bookstore and why the old shop-keeper isn't willing to help them. After all, it was his magical box that transformed Harry into the hideous creature in the first place. Join Harry and his friends on their adventure, but be warned; Gluey Glubber monsters aren't known for their patience and they get Very hungry. There's just no telling what they'll eat next... It Might Even Be You!