I thought Poison Study was just okay, 2.5 stars maybe. Most of the book was slow with little direction. I think the "plot," if one existed, was that Yelena was trying to survive, but she really didn't know how to go about doing that. So mostly, she just ran away and/or fought when she was attacked. I never really felt like I was pulled in to the book, because Yelena had no real plan, purpose, or goals. Her entire personality has that she had a tough life, and she survives. But there was simply not enough depth there for me to really care what happened to her.
Valek did not have much personality either, and I have to admit to being a bit dumbfounded about why they like each other. They did not have any real chemistry--mutual respect, but definitely not romantic chemistry. I could understand Yelena having a teacher-student type innocent crush on Valek, but when did they fall in love?
This book is categorized as fantasy, but the magic system was never described in any detail. And the use of magic was pretty sparse throughout. I am a fantasy fan, and frankly, I feel as though I was tricked into reading this book.