An important book. An "easy" read (at least, in terms of the writing, if not the content). Highly recommended.
On the other hand, the redeeming feature of the book for me was the deeply beautiful romance and heartmate connection between Yelena and Valek. In its own small way, it was more romantic than a great many romances I've read. Their gifts of self to each other (mostly, since it's told from Yelena's POV, Valek's gifts and presence to her) were only occasional, but were the factor that made reading the whole story worthwhile.
Poison Study is wonderful. The characters are life-like and compelling. The story is told first-person from Yelena's perspective, and I feel like her friends are my friends and her enemies are out to get me, too. The author's use of 1st person POV is really well done and to great effect. We only know what Yelena knows, although (especially on my second reading) sometimes Yelena sees things that allow the reader to figure things out before even Yelena does (i.e. the clues are there to know slightly more than she does). The universe, while another variant of Middle Ages with magic, is realistic, holding together in its own way. The content around learning to be a food taster checking the Commander's meals for poison is fascinating and seems really well-researched. (Spoiler alert!) The romance between Yelena and Valek seemed to appear a bit too quickly to me on my first read through, but I could follow the smaller steps better the second time and it wasn't quite such a shock.
Overall a really wonderful and fascinating book. Not a challenging one, just a really good read.