Noughts & Crosses by Malorie Blackman

Sephy is a Cross: she lives a life of privilege and power. But she’s lonely, and burns with injustice at the world she sees around her.

Callum is a nought: he’s considered to be less than nothing – a blanker, there to serve Crosses – but he dreams of a better life. They’ve been friends since they were children, and they both know that’s as far as it can ever go.

Noughts and Crosses are fated to be bitter enemies – love is out of the question. Then – in spite of a world that is fiercely against them – these star-crossed lovers choose each other. But this is love story that will lead both of them into terrible danger… and which will have shocking repercussions for generations to come.
This summer, the teenagers are determined to discover the truth at the heart of their family. But some secrets are better left alone.

Then another girl disappears. As Ellery and Malcolm race to unravel what happened, they realise every secret has layers in Echo Ridge.