Noughts & Crosses by Malorie Blackman
Sephy is a Cross - a member of the dark-skinned ruling class. Callum is a nought - a 'colourless' member of the underclass who were once slaves to the Crosses. The two have been friends since early childhood. But that's as far as it can go. Against a background of prejudice, distrust and mounting terrorist violence, a romance builds between Sephy and Callum - a romance that is to lead both of them into terrible danger . . .