Artemis Fowl by Eoin Colfer
Artemis Fowl, a child prodigy whose intelligence is matched only by his gift for nefarious schemes, begins his escapades through the techno-fantasy world of the fairies with the simple goal of stealing a butt-load of gold. Through a series of adventures with a killer cast of bad guys ranging from a maniacal tech-company CEO to an egomaniac demon from another dimension, Artemis transforms both emotionally and physically and by the end of the series the teenage criminal mastermind ultimately learns who he really is.
The anti-hero is joined by a flatulent dwarf who can’t stop stealing, a sassy captain of the fairy police with a collection of gadgets to make Batman green with envy, and his lifelong bodyguard who has the skill set of a ninja and the disposition of a golden retriever. The series has it all: action, sci-fi, magic, and a dash of romance.
Full disclosure, I have read this series several times. I go back to them when I’ve read something disappointing because if I want to remember how much fun reading can be, I pick up an Artemis Fowl book. A classic middle fiction series that delivers in full, for ages 9+.