This is a MUST-READ for anyone who has read some or all of the Throne of Glass series or who is thinking of starting it. It's debatable at what point in the series it is best to read this. The author suggests before starting Throne of Glass, and others have suggested before reading Empire of Storms. I definitely agree with reading it before Empire of Storms, as there are a lot of cameos from characters that are in the novellas found in The Assassin's Blade. Personally, I would STRONGLY suggest to maybe read it between Heir of Fire and Queen of Shadows because much of this is revisited in QoS and EoS, and I think that reading it before Throne of Glass may spoil the organic way you are introduced to Calaena and her past in the first two books. 
 It's a fairly quick read, as it is made up of 5 novellas that are set in the year before Calaena goes to Endovier. Spoiler for those who haven't yet begun reading the Throne of Glass series:(view spoiler)[ It's a different Calaena than the one we meet in Throne of Glass and it explains the events that are frequently referenced in the rest of the series, including her trip to the Red Desert to train with the Silent Assassins, how she meets the pirates, her relationship with Sam, and the downfall of her relationship with Arobynn. All of these events set into motion her eventual imprisonment in Endovier, where we meet Calaena at the beginning of Throne of Glass. (hide spoiler)]