Crown of Midnight by Sarah J Maas

From the throne of glass rules a king with a fist of iron and a soul as black as pitch. Assassin Celaena Sardothien won a brutal contest to become his Champion. Yet Celaena is far from loyal to the crown. She hides her secret vigilantly; she knows that the man she serves is bent on evil.

Keeping up the deadly charade becomes increasingly difficult when Celaena realizes she is not the only one seeking justice. As she tries to untangle the mysteries buried deep within the glass castle, her closest relationships suffer. It seems no one is above questioning her allegiances—not the Crown Prince Dorian; not Chaol, the Captain of the Guard; not even her best friend, Nehemia, a foreign princess with a rebel heart.