The Last Mapmaker by Christina Soontornvat

Sai has a secret. As Assistant to Mangkon's most celebrated mapmaker, Paiyoon Wongyai, she plays the part of a well-bred young lady with a glittering future. In reality, her father is a low-life con man - and in a kingdom where the status of your ancestors dictates your social position, no one can know that Sai's connection to the past is a single tarnished link. So when Sai is given an opportunity to join Paiyoon on an expedition to chart the southern seas, she jumps at her chance to sever that link and start a new life.

Soon she realizes that there is more to the expedition than meets the eye and that she's not the only one on board who has secrets. When she learns that the ship might be heading for the fabled Sunderlands - a land of dragons, dangers, and riches beyond imagination - she has to decide exactly what she's willing to risk to chase after her dreams.