I wasn't aware until I read this book that Tolkien has become a sufficiently widespread and intensive subject of study to have an academic journal entirely devoted to him and his works! As a direct result of this, we are presented with this book which is not part of Christopher Tolkien's apparent obsession with his father's imagined mythos, being instead edited by Verlyn Flieger.
It's a short, unfinished, stylistically dreadful tale that no sane person would publish alone based on its literary merit - so what's the point? The cynical might argue that for some time now the Tolkien estate has been milking a cash cow that is aging and drying up, producing lower and lower quality product. That may be so, but I don't think Flieger's motivations are cynical at all. This story, which is a very early example of Tolkien's creative output, appears to be a "missing link" between admiration of an existing work (The Kalevala) and inspiration for his own imagined work, specifically the Tale of the Children of Hurin and more generally the Silmarillion as a whole and Flieger sets out to demonstrate this. Hence this book is not really "Here's a forgotten story by the most influential fantasist in history, it's really good!" so much as, "If you're sufficiently interested in Tolkien, his imagined world and creative process, this little, badly written adaptation of a little-known (in English) Finnish folk-tale is important and you should have the opportunity to learn about it."
In that context, this book is worthwhile. Additionally, the biographical aspects of Tolkien's life that raise the personal parallels and significance this story would have had for its author are made clear along with how these developed into what I believe is Tolkien's best story (the above mentioned Children of Hurin). However, even with the attendant notes and essay, one would still have only a very small book. Bulking out the volume (to still very modest proportions) are two versions of an informal lecture on the Kalevala, the source material for the Story of Kullervo and it is this connection that made me interested in this book.
I read a translation of the Kalevala, a collection of Finnish folk ballads assembled into a vaguely narrative sequence, not long after the release of this book and it was amazing! To learn that it was a heavy influence on Tolkien was fascinating and here is the book that is going to tell me what the influences were and what he thought about the source material. So this book may not be for you; if you just want a good story - forget it. If you don't know or don't care about the source material - probably not that interesting. If you don't care about how Tolkien's justifiably famous works came into being - not worth your time. But for me - though the story itself was the least rewarding aspect - well, I ripped through the supporting material in no time, even if the material it supports had me plodding like I was trying to find my way through the Finnish bogs of its setting.