Half Moon Investigations by Eoin Colfer

From chapters.ca:
Ever since he can remember, 12 year old Fletcher Moon has had to suffer the nickname Half Moon because of his short stature. However, what Fletcher lacks in height he makes up for in brains, because Fletcher Moon is the youngest qualified private detective in the world. To achieve this extraordinary feat, Fletcher changed his age on his birth certificate and signed up for an Internet diploma course. He graduated top of his class and received a gold plated detective's shield for his effort. Armed with his prized shield, Fletcher takes on various petty crime cases in his school. Things are going pretty well until one day his shield is stolen and Fletcher is caught up in an investigation involving the town's biggest crime family. Grievous bodily harm, arson, and dangerous undercover work ensue as Fletcher is framed for several crimes he did not commit. He is forced to go on the run from the authorities with twelve hours to find the guilty party - or he is the guilty party.