The plot of this book can be summarized in the following two sentences. Moiraine and Siuan Sanche learn a great secret while still being Accepted in the White Tower. Moiraine meets Lan. There are much more than these events in the prequel and some more familiar faces appear than the ones I already mentioned.
I gave this book 5 stars initially, but I was sure I would lower the rating to 4 stars after I read this tale for the second time. To my complete surprise I realized that is still worth 5 stars. I think the good prequels must show how some of the relationship already developed in the main storyline came to be and some insights on their development. In this regard the book succeeds - no questions about it. It also happened to have interesting and fast-moving (in the second half of the book) plot.
The first half of the book is mostly spent on Moiraine and Siuan Sanche training in the White Tower. While this part is somewhat slower, I still find it interesting. I always wondered what the test for full Aes Sedai looks like: we are shown test for Accepted from both Nynaeve and Egwene POV, but never the former. This book gives Moiraine's POV during this test as well as preparation for it.
Robert Jordan once again showed his strength as a worldbuilder: he gave interesting glimpses of Malkieri culture: original and unique; in the first ten books Lan was the only Malkieri we get to see.
The prequel totally deserves 5 stars as it completely fulfills its purpose while being interesting. One last piece of advice: the book contains minor spoilers; it is better to read it in publication order (after book 10).
This review is a copy/paste of my BookLikes one: http://gene.booklikes.com/post/814562/surprisingly-this-prequel-is-not-useless