Sister by Rosamund Lupton, one of my Oxfam finds. I loved the Hunger Games books.
I've just finished final book of the Veronica Roth trilogy (Divergent, Insurgent and Allegiant) which are kind of similar and very good. The film of the first one is out in a few months.
Before that, Dorothy Koomson's Marshmallow's for Breakfast which was great, I love her books.
Before that, Lake of Dreams, by Kim Edwards (The Memory Keeper's Daughter author). Another Oxfam find, I find it great for a browse and just pick up random books which catch my eye.