The Infinite Sea, and it comes from E. Lockhart, the author of The Disreputable History of Frankie Landau-Banks, a smart, funny, feminist YA that I loved.
We Were Liars is also different than Disreputable History, as it's centered on a mystery and potentially unreliable narrator. Its narrator is Cadence, the oldest grandchild of the Sinclairs, a wealthy family that summers on their own island. The three aunts (including Cadence's mother) spend most of the summer sniping over the family wealth, but Cadence and her cousins Mirren and Johnny--as well as Johnny's Indian friend Gat--don't want anything to do with the family conflict. The book takes place two summers after a terrible accident that left Cadence with selective amnesia, and the novel's focus is on the process of Cadence unraveling what really happened.
Lockhart does a great job of building up the suspense around what happened to Cady, and she casts doubt on what all the characters have to say by including the family conflict and by telling us Cady and her cousins are collectively called "The Liars." The mystery drives the book at breakneck speed (I read it in one afternoon). The mystery is theoretically complemented by the romance between Cady and Gat, but I never felt much connection between them.
But, most unfortunately, Lockhart's reveal isn't able to live up to the tension of most of the novel. The "truth" isn't especially interesting or shocking (especially when the book's blurb summary sets huge expectations by proclaiming, "And if anybody asks you the ending, just lie."). And I never got why the group was called "The Liars." They didn't seem to lie all that much, and Cady's narration appeared to be truthful.
We Were Liars was still a good ride with a dud ending.