Katy Rivers' life finally seems to be picking up. She's dating Merlin, her school's resident artist hottie and has a great group of friends. However, when she sees a strange girl while on the bus with piercing green eyes, things begin to unravel. Add in a mysterious green pendant she receives from her indirectly and Katy starts to wonder if this new girl, Genevive, is entirely human and if she wants to take over Katy's life.As Genevive worms her way into Katy's life through her friends, mother and even her boyfriend while appearing perfectly sweet to them, and saying she will is Katy's worst nightmare to her in private. Katy finds herself in a battle for her friends and trying to prove that this girl is not all she seems.What I most loved about this novel is that it consistently blurs that line between reality and fiction. Katy is convinced there is something otherly about Genevieve, but her best friend Luke is certain she is bestowing supernatural qualities on someone completely normal. This created real psychological tension as you were never sure who exactly was right. In a world of paranormal young adult, this uncertainty was welcome to me.My favourite character was definitely Luke. I felt his friendship with Katy was really sweet and I loved the subtle growing attraction that developed in the novel. There is a small age-difference between the two, but to be honest I know a lot of people who have dated and been good friends with the same disparity so I didn't find it a big deal. I loved that Luke was the person who believed and supported her throughout the novel and I just wanted to be friends with him from the moment he entered the novel. Katy and him had a natural chemistry and very believable friendship that was my personal favourite feature of Poison Heart.I also loved how much Katy developed and changed in the novel, becoming much more independent and assertive by the end. Her metamorphosis was very believably written and happened subtly throughout the novel.In terms of the mystery aspect, I guessed some of it but certain aspects did come as a surprise for me. The tension and suspense is well written and to that end, I think you will either love or hate the ending because of that. I would recommend this novel to fans of suspense novels and YA, as well as those who want a fresh twist on paranormal novels.S B Hayes evokes a compelling psychological thriller with just a hint of the paranormal and is a welcome addition to the British YA scene. Poison Heart is due out today in the UK, published by Quercus. I received the book from the publishers for free (thank you!) and am very happy I got a chance to read and review this novel. My review is not affected by how I received the book in the slightest.