Malorie Blackman 
Noble Conflict 

Years after a violent war destroyed much of the world, Kaspar has grown up in a society based on peace and harmony. But beyond the city walls, a vicious band of rebels are plotting to tear this peace apart. It is up to the Guardians – an elite peacekeeping force – to protect the city, without ever resorting to the brutal methods of their enemy.
When Kaspar joins the Guardians, he has a chance encounter with a rebel – a beautiful girl named Rhea. Haunted from that moment on by strange visions and memories – memories that could only belong to Rhea – he realises he hasn”t been told the truth about what the rebels really want, and what he”s really fighting for.