Artemis Fowl by Eoin Colfer

No scheme is too dastardly, no plot too devious. And he's just discovered that fairies are real.

Poor fairies.

But these are not the cuddly creatures of bedtime stories. They are armed. They are dangerous. And when Artemis captures Captain Holly Short for her fairy gold, he messes with the wrong elf.

Holly isn't armed but she's incredibly dangerous, and pretty annoyed with all the kidnapping.