I have a soft spot for fantasy books that have a medevil type of setting. I also have a very soft spot for tall dark handsome men (with assassin type tendancies) - so in short I loved Poison Study and I'm in love with Valek.
Yelena is a rather likeable character - as you learn more about her and her past you feel a certain amount of empathy for her - due to the torture and abuse she has dealt with. The romance angle is dealt with rather softly, with Valek as her boss she initially resents him, but soon starts to miss him when he's not with her and eventually realises she loves him. For Valek - who knows, but certainly during his 'test' it becomes clear that he has some feelings (although he's rather good at hiding them).
Ari and Janco were such good fun, and it's easy to see why Yelena considers them family by the end of the book (definately older brother material).
I'm excited to see what happens in Magic Study!!