Throne of Glass [paperback] by SarahJ.Maas

'One of the best fantasy book series of the past decade' TIME
From the #1 bestselling author of A Court of Thorns and Roses, Throne of Glass is the first book in the epic, bestselling fantasy series that has captivated readers worldwide.
Enter the realm. Unleash the darkness. Live the legend.
In a land without magic, an assassin is summoned to the castle. Celaena Sardothien has no love for the vicious king who rules from his throne of glass, but she has not come to kill him. She has come to win her freedom. If she defeats twenty-three murderers, thieves, and warriors in a deadly competition, she will be released from prison to serve as the King's Champion.
But something rotten dwells in the castle and it's there to kill. When her competitors start dying mysteriously, one by one, Celaena's fight for freedom becomes a fight for survival and a desperate quest to root out the evil before it destroys her world, and the people she has come to love.