A Sorcerer's Treason introduces the reader to Isavalta by dumping its heroine Bridget into a situation of court intrigue in a magical land. Along the way, A Sorcerer's Treason fills the reader in with all of the Isavalta background - but apparently Zettel was so enamored with that background material that she decided to retell that story in the second book, Usurper's Crown. I confess I was confused for a while as to how Usurper's Crown tied into A Sorcerer's Treason - I was anticipating that the second book would advance the plot beyond the point where the first book left off. Instead, when the Usurper's Crown ends, the reader is no further along in the story line than at the end of A Sorcerer's Treason. Traditionally, the middle book of a trilogy is the weakest, but it is unusual to find a trilogy where the second book doesn't advance the story along even a little. And since A Sorcerer's Treason already explained the preceding events, the retelling of the tale is not tremendously enjoyable read.
This is not to say that Usurper's Crown is a bad book. Had I read it as a stand alone novel, I would certainly have rated it four stars. The romantic / emotional interaction between Avanasy and Ingrid is quite well done, and all the characters are believable. The system of magic, which seems to be based upon weavings and knots seems interesting. The land is well described. I want to learn more about the Vixen and Babba Yagga, but unfortunately this book does nothing to further illuminate the conflict between them.
This book rates three stars because it is well written and a fine read, but you can skip it if you have already read A Sorcerer's Treason.