Fills in mostly anecdotes about Moiriane, Lan & Siuan. It does not have to be read to enjoy the WoT series as a whole.
I really enjoyed reading this book, but I suggest to read it after having read the first couple of books of the series.
It is a nice insight of Moiraine and Lan and how they met each other.
Although it is mostly centered on Moiraine, you get a feeling of the background of both of them.
I liked to read how the White Tower teaches to Novices and Apprentices.