The 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, Tane has trained to be a dragonrider since she was a child, 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.
 
An enthralling, epic fantasy about a world on the brink of war with dragons - and the women who must lead the fight to save it.
 
A Day Of Fallen Night, another book in the Samantha Shannon's world of the Prioriy is now available for pre-order.
 
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 'Escapism at its finest -- Shannon, we salute you' - Stylist
 
'Epic' - Guardian