We Were Liars by E. Lockhart

Here's the official description:
A beautiful and distinguished family. A private island. A brilliant, damaged girl; a passionate political boy. A group of four friends -the Liars- whose friendship turns destructive. A revolution. An accident. A secret. Lies upon lies. True love. The truth. Read it. And if anyone asks you how it ends, just LIE.

And the first page because why not:
Welcome to the beautiful Sinclair family. No one is a criminal. No one is an addict. No one is a failure. The Sinclairs are athletic, tall, and handsome. We are old-money Democrats. Our smiles are wide, our chins square, our tennis serves aggressive. It doesn't matter if divorce shreds the muscles of our hearts so that they will hardly beat without a struggle. It doesn't matter if trust-fund money is running out; if credit card bills go unpaid on the kitchen counter. It doesn't matter if there's a cluster of pill bottles on the bedside table. It doesn't matter if one of us is desperately, desperately in love. So much in love that equally desperate measures must be taken. We are Sinclairs. No one is needy. No one is wrong. We live, at least in the summertime, on a private island off the coast of Massachusetts. Perhaps that is all you need to know.