IMHO this was an excellent series and this book was perhaps the best of the three.
The two main characters, Mia and Loie, are teenagers who were borne on the same day, in the same hospital and have literally been friends their entire lives. Loie is an introvert and Mia is an extrovert. Loie is poor and Mia is very well to do. Mia believes she will inherit some unknown special powers from her father when she turns 17. Loie has forever been Mia's support and Mia has forever been Loie's shelter. Loie's parents died when she was 5 years old and Mia's father disappeared that same day. I especially loved the characters and how they developed throughout the series. Being teens they have angst and happen to both fall in love with the same boy, Andreas. In book two Cian (pronounced Keane) is introduced as a possible love interest. Neither Andreas nor Cian are who they initially appear to be.
They live in a world where there are evolved humans referred to as Universals and others referred to as Ancients. The Ancients are a collection of fae, sprites, pixies, gods, and various other mythological beings. They're sealed away in another dimension known as the Yawning Void. The Universals live in a place known as Sanctuary. Humans cannot travel to these places but he Universals and the Ancients can and do come to earth. While not all Ancients are "bad", many are and so they've been sealed away in the YV but manage to come through gateways to get to earth.
In book three the trilogy completes with a visit to the Yawning Void that brings surprises of its own.
I don't want to be guilty of spoilers so I will stop here. I loved this series and hope to find more like it in the future. I highly recommend this to anyone who enjoys reading a good book about paranormal type activities. This didn't feel so much like a teen book as it did a really good story with teenaged characters. There were plenty of adults with strong roles as well and it would make a very good family movie.