The Wind Singer (Wind on Fire trilogy, #1)

Kestrel Hath's schoolroom rebellion against the stifling caste system of Aramanth leads to explosive consequences for her and her family: they are relegated to the city's lowest caste and are ostracized. With nothing left to lose, Kestrel and her twin brother, Bowman, do the unthinkable: they leave the city walls. Their only hope to rescue the rest of their family is to find the key to the wind singer, a now-defunct device in the city's center, which was once the course of happiness and harmony in Aramanth. But the key was given to an evil spirit-lord, the Morah, in exchange for the Morah's calling off its terrible army of Zars. Armed with desperate bravery, wits, and determination, Kestrel, Bowman, and a tagalong classmate set off to find the key. Along the way they meet allies and foes, but in order to succeed in their quest, they must face the most sinister force of all: the powerful Morah.