This. Book.
I picked it up from Waterstones on its release date, and was intrigued to see whether the endless hype that I'd heard about it was right; and it really, really was.
We Were Liars is a book that the less you know about it, the better it will be. All I will say is that its told from the perspective of Cadence Sinclair, who is a member of a rich old family who pretty much own an island, where they all spend their holidays together. Cadence spends her summers with her cousins; Johnny, Mirran and Gat. One year, Summer Fifteen, something happens which makes her ill and causes her not to be allowed to visit the following year. So in her next summer, Cadence attempts to put together what happened.
Lockhart's writing is wonderful. I cried. It's really fantastic. And I'm currently placing an order to buy Lockhart's previous works because I am currently a little bit in love.