I'm quite surprised there isn't a more recent post about this series, I absolutely love them and I guess I assumed they were more popular then they are? The series is:
Noughts and Crosses
Knife Edge
Checkmate
Double Cross
They're for young adults, I read the first one when I was 13 and still love the entire series now, at 19. I would recommend them a thousand times over, they've got an element of everything and are so thought provoking. For me, they were one of those series that stays with you for a long, long time after you've finished them and can easily be read many times.
Without giving too much away, the first book is the story of Callum and Sephy and I would argue it's first and foremost a tragic romance with serious issues challenging them. Such as, racism and terrorism (or freedom fighters...depending entirely on your perspective).
The rest of the books follow the story of Callum and Sephy and then the narrator changes towards the end (I don't want to say who to, it may spoil the story!).
So, if you're looking for a new series which is as amazing as Twilight (in my opinion) yet deals with issues that are very much present in today's society, then you should buy this. It's amazing