Half-Moon Investigations, by Eoin Colfer, 2006

Fletcher Moon is an official detective with a badge, even if he is only twelve. When popular April Devereux hires Fletcher to investigate the theft of a lock of celebrity hair, the case gets personal. And Red Sharkey, the charismatic scion of the notorious criminal Sharkey family, has reasons of his own for making sure "Half-Moon" solves the case. Hilarious middle-grade detective fiction, set in in Ireland, with Colfer's trademark ratcheted stakes. Strongly recommended.