The Assassin and the Desert by Sarah J. Mass

The Assassin and the Desert (Throne of Glass, #0.2) - Sarah J. Maas

In this second short story, Celaena is sent to the Red Desert as a punishment, to attend to the Silent Assassins, where she needs to train with them and bring home a recommendation letter from their master. Celaena is used to being alone, taking care of herself and not needing anybody, but there she meets Ansel, another great assassin. Ansel is a girl damaged by her past and I loved how the two girls bonded over this and became close friends. But someone is plotting with Silent Assassin’s enemies and Celaena finds herself in the midst of the fight. This is a story about friendship and betrayal. I really didn’t see the big twist coming. The things were solved a bit too easy for me, but I was happy to finally see Celaena and her assassin skills in action.

Source: http://thetemporaryescape.blogspot.com/2013/09/mini-reviews-prequels-to-throne-of.html