Arthur and the Invisibles (Film Tie-in EDN) by Luc Besson
Do the Minimoys really exist? Arthur has read all about the tribe in his grandfather's notebooks, but he can hardly believe they are less than an inch tall. Until one day, he discovers a secret message and a map to the land of the Minimoys, hidden by his vanished grandfather, which sets him on a wild adventure. Arthur has three days to discover the Minimoys, recover stolen treasure, fight a force of mosquito-riding warriors, defeat an evil wizard and return to save the family home. All the more difficult if you have been shrunk to the size of a Minimoy...