The Fowl Twins (3 book series) by Eoin Colfer

One week after their eleventh birthday, the Fowl twins–scientist Myles, and Beckett, the force of nature–are left in the care of house security (NANNI) for a single night. In that time they befriend a troll who has clawed his way through the earth’s crust to the surface. Unfortunately for the troll, he is being chased by a nefarious nobleman and an interrogating nun, who both need the magical creature for their own gain, as well as a fairy-in-training who has been assigned to protect him. The boys and their new troll best friend escape and go on the run. Along the way they get shot at, kidnapped, buried, arrested, threatened, killed (temporarily), and discover that the strongest bond in the world is not the one forged by covalent electrons in adjacent atoms, but the one that exists between a pair of twins.