We Were Liars by E. Lockhart

Cadence Sinclair Eastman, the eldest grandchild of the affluent Sinclair family, spends her summers on their private island, Beechwood. She forms a close-knit group with her cousins Johnny and Mirren, and family friend Gat; together, they're known as "the Liars."

During the summer when Cadence is fifteen, she suffers a traumatic head injury, resulting in amnesia and debilitating migraines. Two years later, she returns to the island, determined to uncover the truth about what happened that fateful summer. As she pieces together her fragmented memories, Cadence confronts the dark realities of her family's legacy and the tragic events that have been concealed from her.