One improvement over Magic Study is that this is more original than the previous tome. The author also seems to make efforts to amend the most Mary Sue aspects of the heroine. So these are welcome changes.
Another good point is that there are humourous dialogues (and even a little situational irony) and they make for a more interesting read.
The plot is interesting, but suffers a bit from the "late explanation syndrome". Instead of giving us clues to unravel the plot, the author keeps the reader and her protagonists in the dark, then later has to introduce new information to shed light on the latest development. So don't be surprised if the denouement comes Deus Ex Machina style. And that's too bad, because the transition from where the previous book ended to where this one starts is quite well done. The expansion on the concept of magic is very neat and fits in with Magic Study. Unfortunately, it also makes Yelena seem a bit like a dunderhead that she still doesn't know her "Magic 101" after spending many months at the Keep, but the reader understands that it's plot dictated, because if she did then almost none of the events would have taken place.
It is too bad that the wonderfully original plot device (the poison tasting) can no longer be used and I don't think anything else the author throws our way really measures up.
Also, the characters are under-developed. Secondary characters, who made the charm of previous installments, are reduced to the literary equivalent of "extras", or worse, they become props to explain how Yelena can affect so many events at once. Even a few villains' backstory remains very sketchy at the end and that makes their motivation seem one-dimensional or ridiculous. And that makes Yelena's triumph seem much less interesting.
Many times, the author makes Leif and other characters say that Yelena is the leader now. This is unfortunate as readers usually prefer to be shown rather than spoon fed. At times, the writing starts resembling the latter.
The emotional conflict is really centered on Yelena. Other characters' angst is downplayed and not given room to mature. And except for Yelena's big woe, conflicts are resolved almost as quickly as they are actually spelled out. I think more actual interaction between the main character and the secondary characters would have been a good opportunity to make these conflicts more compelling.
Since the book focuses so much on Yelena, reading it becomes somewhat tedious at a point where Yelena isn't so smart or so nice. It picks up near the end to come to the conclusion but the pacing is uneven and doesn't maintain the reader's attention throughout.