David Whitley 
Midnight Charter [EPUB ebook] 

In the city of Agora, anything can be bought and sold. Even children are possessions until their twelfth birthday.Mark has been sold by his father, and Lily, an orphan from birth, has bartered for her life. Thrown together by chance, in the ancient tower of Count Stelli, they face an existence of poverty and servitude, unless they can find a way to break free. But, unbeknown to Mark and Lily, they are being watched by the ruler of the city. Can they survive the traps and treachery that await them and discover the dark secret that binds them together?Their lives depend on this question: what is the Midnight Charter?