My first question is: when is the next one coming out? I love this series and I love this book.
You jump straight into the action with this book as is normal for a Elemental Assassin book. This book gives more insight into Owen's past and Gin even makes a new friend, of a sort. He's even helpful!
This book focused a lot on Owen and his life before he met Gin, mostly his teenage years. Owen's parents were also killed by Mab (like Gin's family was). He had to take care of his little sister, Eva, from the time he was 15 (she was 2) on. Salina is part of this past and when she comes back to Ashland, trouble starts: both in Owen and Gin's relationship and in Ashland as a whole.
Salina is a powerful water elemental who's father was killed by Mab (seeing a pattern, here?). You learn a lot in this book about Eva, as well, but I won't spoil by giving it away.