I read this novella as an extra bonus the UK paperback of Blood Moon.
Lost Girls tells the story of how Liam and Helena first met. I don't want to say much about the story because it would be too easy to give spoilers but I really enjoyed getting to see them both before they married and had the children. It's easy to see them just as parents in the rest of the series and not think about the lives they had before they met so I liked this extra side to them. It was also great to get a better idea of how and why Helena became the kick ass mum that she is now.