Priory Of The Orange Tree by Samantha Shannon

A world divided.
A queendom without an heir.
An ancient enemy awakens.

The House of Berethnet has ruled Inys for a thousand years. Still unwed, Queen Sabran the Ninth must conceive a daughter to protect her realm from destruction--but assassins are getting closer to her door.

Ead Duryan is an outsider at court. Though she has risen to the position of lady-in-waiting, she is loyal to a hidden society of mages. Ead keeps a watchful eye on Sabran, secretly protecting her with forbidden magic.

Across the dark sea, Tané has trained all her life to be a dragonrider, but is forced to make a choice that could see her life unravel.

Meanwhile, the divided East and West refuse to parley, and forces of chaos are rising from their sleep.