Sarah J. Maas 
Assassin and the Underworld [EPUB ebook] 
A Throne of Glass Novella

A look at Celaena Sardothien’s life before the events of Throne of Glass.When the King of the Assassins gives Celaena Sardothien a special assignment that will help fight slavery in the kingdom, she jumps at the chance to strike a blow against an evil practice. The misson is a dark and deadly affair that takes Celaena from the rooftops of the city to the bottom of the sewer-and she doesn’t like what she finds there.