Full review can also be found on: Paein and Ms4Tune's Book Blog 
 While billed as a prequel this actually happens after the start of The Shadow Soul as it's set roughly only a week before the Prince Rhen arrives in Ourthuro. Therefore I'd recommend it be read AFTER The Shadow Soul. You're also likely to care more about the characters. This is a sad story. It's followed almost immediately by The Silver Key.