Book Spotlight: We Were Liars by E. Lockhart

Cadence Sinclair Eastman is the eldest grandchild in an extremely wealthy, tight-knit family run by her domineering grandfather. She spends every summer on his private island off the coast of Massachusetts with her favorite people in the world: her cousins Mirren and Johnny, and Johnny's best friend Gat. The summers always have a wonderful predictability when the four teenagers are reunited, full of long days on the beach, games, and adventure. But the summer of her fifteenth birthday is different in every way. Cadence finds herself falling in love with Gat, and when a terrible accident happens she is left with a severe head injury and amnesia. 

After the accident, it is two years before Cadence returns to the island and sees Mirren, Johnny, and Gat, and everything feels different. As she slowly tries to piece together what happened the summer she was fifteen, she is completely unprepared for what she will find out.