Before I go wandering off into criticism and you think I'm dissatisfied, let me state that I really enjoyed the series, and am pleased that each book was better than the previous one. When my niece is ready, I will be happy to get all three for her. Or maybe I'll give them to my sister for her birthday in the fall.
The main character, Yelena, is much like the heroine Kaylin from the "Cast in..." series by Michelle Sagara - muleheaded, strong, and a realist to the point of tunnel vision. Like Kaylin, Yelena builds her determination and relationships on the grounds of belief where there is none, and also rejects knowledge and learning in any regular fashion - the cause of most of her problems along the way to resolving the plot-problem.
The Poison/Magic/Fire Study books are not spectacular examples of literary prose, but rather of literary craftsmanship. Snyder's worlds are as distinct as the regions in the movie version of Lord of the Rings. Snyder uses color, animals, smells, even the way the wind blows to make sure that the details of each location and action are in focus and targeted to where the reader's attention should go. Unfortunately, she waxes and wanes much like Robin McKinley, so that some of her scenes are lush and full, and some of the action scenes are distilled into a three line conversation among characters. And yes, some verbs are used far too often. Look, everyone has a favorite toy, but at times that switchblade got more press just for being in her pocket than her horse did for being a combatant.
The biggest flaw I kept tripping over while reading Fire Study was the repetitiveness of the characters' irritations (again, same as with Sagara's series). Ok, so Moon Man's cryptic remarks annoy you. Got it, let's move on... The least interesting side plot was having the characters finally realize who the main antagonist was (we already knew from the beginning of the book). And the bat was just annoying.
For a hopefully-final book in the trilogy, I was very pleased with which plots were finalized and which were not. Fisk got to grow up to be a bigger character, though Dax was dropped from plot-usefulness. The most interesting side plot was the final explanation of the Commander, and it was well timed/crafted to allow a verbal summary of most of what Yelena had learned about her magic.