There was so much going on with this book... I don't even know where to start. There are pures and half-bloods. In relation to normal human lives, pures are like the celebrites who get everything just for being themselves and half-bloods are even worse off than middle-class. Most half-bloods are forced into a drug-induced slavery working for the pures unless they have a rich pure to back them up and send them to school. The half-blood slaves are literally drugged routinely to keep them calm and complacent.
So Alex is a half-blood that had the opportunity to go to school, but was pulled out by her mother. They both fled the safety of the islands that the pures and half-bloods live on and hid out in various places for three years. Her mother is killed by a daimon, which are basically shadow-like demons who bite pures and half-bloods to drink the ether (life force) that's in their blood. So in comes Aiden, the impeccabbly hot pure blood to save Alex and bring her back to the school.
From there, the story builds on Alex's determination and passion to become a Sentinal and we get to see the relationships she has with those who surround her. I've always been attracted to anything involving mythology and this book was no different. It was literally one of my favorite reads of the year. So basically, you should go buy this.