What's cool about these novellas, is that the five of them together make a prequel for Throne of Glass. Each novella builds upon the previous one, and they don't feel like filler or fluff which can happen with novellas sometimes, they feel like a complete novel. These novellas give so much insight in Celaena's character and her past. In this book we meet Sam, Celaena's first love and Arobynn, the Assassin King who has trained Calena since she was a young girl and learn more about the betrayal that landed Celaena into Endovier.
This book was very fast paced, just like Maas' other novels, and I would recommend it to those who read the first two books and need more Celaena before the third book, Heir of Fire comes out in September. (which by the way has another gorgeous cover) These novellas really add to the series, and although I don't think you'll be totally lost if you don't read them, they are very enjoyable to read and do add to the series and the character development.
Throne of Glass Review
Crown of Midnight Review